snap-<snapname>-<revision>.mount systemd units left behind

`snap` evidently automatically updates snaps, but in the process of doing so, leaves old `snap-<snapname>-<revision>.mount` systemd units left behind in memory. Because the unit definitions are no longer on disk in /etc/systemd/, this leads `systemctl` to report that the system is degraded.

Contents of `/etc/systemd/system`:

```
-rw-r--r-- 1 root root 285 Oct 18 02:01 snap-core18-2128.mount
-rw-r--r-- 1 root root 285 Oct 26 08:46 snap-core18-2246.mount
-rw-r--r-- 1 root root 285 Oct 13 13:49 snap-core20-1169.mount
-rw-r--r-- 1 root root 279 Oct 26 22:47 snap-lxd-21780.mount
-rw-r--r-- 1 root root 279 Nov 10 09:57 snap-lxd-21858.mount
-rw-r--r-- 1 root root 285 Oct 20 06:10 snap-snapd-13640.mount
-rw-r--r-- 1 root root 285 Nov 9 23:47 snap-snapd-13831.mount
-rw-r--r-- 1 root root 288 Oct 18 02:02 snap-wormhole-349.mount
```

`$ systemctl status` shows 3 units failed (2 are lxd and 1 is snapd):

```
● ubuntu
    State: degraded
     Jobs: 0 queued
   Failed: 3 units
    Since: Tue 2021-10-26 16:31:38 UTC; 2 weeks 3 days ago
...
```

 `$ systemctl -a snap-snapd*` yields 1 failure:

```
● snap-snapd-13640.mount - Mount unit for snapd, revision 13640
     Loaded: loaded (/proc/self/mountinfo; enabled; vendor preset: enabled)
     Active: active (mounted) since Tue 2021-10-26 16:31:46 UTC; 2 weeks 3 days ago
      Where: /snap/snapd/13640
       What: /dev/loop4
      Tasks: 0 (limit: 231992)
     Memory: 144.0K
        CPU: 8ms
     CGroup: /system.slice/snap-snapd-13640.mount

Oct 26 16:31:45 ubuntu systemd[1]: Mounting Mount unit for snapd, revision 13640...
Oct 26 16:31:46 ubuntu systemd[1]: Mounted Mount unit for snapd, revision 13640.

× snap-snapd-13270.mount
     Loaded: not-found (Reason: Unit snap-snapd-13270.mount not found.)
     Active: failed (Result: exit-code) since Tue 2021-11-09 23:47:38 UTC; 3 days ago
        CPU: 7ms

Oct 26 16:31:45 ubuntu systemd[1]: Mounting Mount unit for snapd, revision 13270...
Oct 26 16:31:46 ubuntu systemd[1]: Mounted Mount unit for snapd, revision 13270.
Nov 09 23:47:38 ubuntu systemd[1]: Unmounting Mount unit for snapd, revision 13270...
Nov 09 23:47:38 ubuntu umount[2461004]: umount: /snap/snapd/13270: not mounted.
Nov 09 23:47:38 ubuntu systemd[1]: snap-snapd-13270.mount: Mount process exited, code=exited, status=32/n/a
Nov 09 23:47:38 ubuntu systemd[1]: snap-snapd-13270.mount: Failed with result 'exit-code'.
Nov 09 23:47:38 ubuntu systemd[1]: Unmounted Mount unit for snapd, revision 13270.

● snap-snapd-13831.mount - Mount unit for snapd, revision 13831
     Loaded: loaded (/proc/self/mountinfo; enabled; vendor preset: enabled)
     Active: active (mounted) since Tue 2021-11-09 23:47:31 UTC; 3 days ago
      Where: /snap/snapd/13831
       What: /dev/loop7
      Tasks: 0 (limit: 231992)
     Memory: 4.0K
        CPU: 6ms
     CGroup: /system.slice/snap-snapd-13831.mount

Nov 09 23:47:31 ubuntu systemd[1]: Mounting Mount unit for snapd, revision 13831...
Nov 09 23:47:31 ubuntu systemd[1]: Mounted Mount unit for snapd, revision 13831.
```

and `$ systemctl -a snap-lxd*` yields 2 failures:

```
× snap-lxd-21750.mount
     Loaded: not-found (Reason: Unit snap-lxd-21750.mount not found.)
     Active: failed (Result: exit-code) since Wed 2021-11-10 09:58:22 UTC; 3 days ago
        CPU: 8ms

Oct 26 16:31:45 ubuntu systemd[1]: Mounting Mount unit for lxd, revision 21750...
Oct 26 16:31:45 ubuntu systemd[1]: Mounted Mount unit for lxd, revision 21750.
Nov 10 09:58:22 ubuntu systemd[1]: Unmounting Mount unit for lxd, revision 21750...
Nov 10 09:58:22 ubuntu umount[3303696]: umount: /snap/lxd/21750: not mounted.
Nov 10 09:58:22 ubuntu systemd[1]: snap-lxd-21750.mount: Mount process exited, code=exited, status=32/n/a
Nov 10 09:58:22 ubuntu systemd[1]: snap-lxd-21750.mount: Failed with result 'exit-code'.
Nov 10 09:58:22 ubuntu systemd[1]: Unmounted Mount unit for lxd, revision 21750.

● snap-lxd-21858.mount - Mount unit for lxd, revision 21858
     Loaded: loaded (/proc/self/mountinfo; enabled; vendor preset: enabled)
     Active: active (mounted) since Wed 2021-11-10 09:58:00 UTC; 3 days ago
      Where: /snap/lxd/21858
       What: /dev/loop3
      Tasks: 0 (limit: 231992)
     Memory: 4.0K
        CPU: 6ms
     CGroup: /system.slice/snap-lxd-21858.mount

Nov 10 09:58:00 ubuntu systemd[1]: Mounting Mount unit for lxd, revision 21858...
Nov 10 09:58:00 ubuntu systemd[1]: Mounted Mount unit for lxd, revision 21858.

● snap-lxd-21780.mount - Mount unit for lxd, revision 21780
     Loaded: loaded (/proc/self/mountinfo; enabled; vendor preset: enabled)
     Active: active (mounted) since Tue 2021-10-26 22:47:29 UTC; 2 weeks 3 days ago
      Where: /snap/lxd/21780
       What: /dev/loop8
      Tasks: 0 (limit: 231992)
     Memory: 4.0K
        CPU: 5ms
     CGroup: /system.slice/snap-lxd-21780.mount

Oct 26 22:47:29 ubuntu systemd[1]: Mounting Mount unit for lxd, revision 21780...
Oct 26 22:47:29 ubuntu systemd[1]: Mounted Mount unit for lxd, revision 21780.

× snap-lxd-21723.mount
     Loaded: not-found (Reason: Unit snap-lxd-21723.mount not found.)
     Active: failed (Result: exit-code) since Tue 2021-10-26 22:47:47 UTC; 2 weeks 3 days ago
        CPU: 8ms

Oct 26 16:31:45 ubuntu systemd[1]: Mounting Mount unit for lxd, revision 21723...
Oct 26 16:31:46 ubuntu systemd[1]: Mounted Mount unit for lxd, revision 21723.
Oct 26 22:47:47 ubuntu systemd[1]: Unmounting Mount unit for lxd, revision 21723...
Oct 26 22:47:47 ubuntu umount[117003]: umount: /snap/lxd/21723: not mounted.
Oct 26 22:47:47 ubuntu systemd[1]: snap-lxd-21723.mount: Mount process exited, code=exited, status=32/n/a
Oct 26 22:47:47 ubuntu systemd[1]: snap-lxd-21723.mount: Failed with result 'exit-code'.
Oct 26 22:47:47 ubuntu systemd[1]: Unmounted Mount unit for lxd, revision 21723.
```