chrony package removal fails

[Impact]

 * I was cherry-picking a fix we needed due to the recent systemd
   changes to timesyncd. There was also a fix-of-the-fix a bit later which
   is missing and therefore breaking package removal.

 * The fix is as simple as adding the "remove" target to the case statement
   on the empty target and thereby avoiding breakage.

[Test Case]

 * apt install chrony
 * apt install systemd-timesyncd
   # this should list and work:
   The following packages will be REMOVED:
     chrony
 * apt install chrony
 * apt remove chrony

[Regression Potential]

 * It is a silly bug due to cherry picking a fix without the fix-of-the-
   fix. I honestly see no regression due to re-adding "remove" target.

[Other Info]

 * This does not necessarily have to be on the -release. Nothing to stop
   generating the ISO at least. But I'd appreciate if we could accept it
   into -proposed asap to hopefully (if we rebuild once) pick it up.
   The reason is that I haven't seen reports, but can think of odd
   dependency chains like
    1. install chrony (will remove systemd-timesyncd)
    2. remove chrony fails
    3. but installing systemd-timesyncd fails as well
   It can be hard to get this resolved and the change is small.
   So I'd appreciate if we could avoid it completely.

root@f:~# apt install systemd-timesyncd
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
  chrony
The following NEW packages will be installed:
  systemd-timesyncd
0 upgraded, 1 newly installed, 1 to remove and 0 not upgraded.
1 not fully installed or removed.
Need to get 0 B/27.9 kB of archives.
After this operation, 290 kB disk space will be freed.
Do you want to continue? [Y/n] Y
dpkg: chrony: dependency problems, but removing anyway as you requested:
 systemd depends on systemd-timesyncd | time-daemon; however:
  Package systemd-timesyncd is not installed.
  Package time-daemon is not installed.
  Package systemd-timesyncd which provides time-daemon is not installed.
  Package chrony which provides time-daemon is to be removed.

(Reading database ... 83798 files and directories currently installed.)
Removing chrony (3.5-6ubuntu5) ...
postrm called with unknown argument `remove'
dpkg: error processing package chrony (--remove):
 installed chrony package post-removal script subprocess returned error exit status 1
dpkg: too many errors, stopping
Errors were encountered while processing:
 chrony
Processing was halted because there were too many errors.
E: Sub-process /usr/bin/dpkg returned an error code (1)

---

In a fresh 20.04 lxc/chroot install, then remove chrony:

# apt install chrony
...
# apt install systemd-timesyncd
Reading package lists... Done
uilding dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
  libfreetype6
Use 'apt autoremove' to remove it.
The following packages will be REMOVED:
  chrony
The following NEW packages will be installed:
  systemd-timesyncd
0 upgraded, 1 newly installed, 1 to remove and 0 not upgraded.
Need to get 27.9 kB of archives.
After this operation, 290 kB disk space will be freed.
Do you want to continue? [Y/n]
Get:1 http://archive.ubuntu.com/ubuntu focal/main amd64 systemd-timesyncd amd64 245.4-4ubuntu1 [27.9 kB]
Fetched 27.9 kB in 0s (951 kB/s)
dpkg: chrony: dependency problems, but removing anyway as you requested:
 systemd depends on systemd-timesyncd | time-daemon; however:
  Package systemd-timesyncd is not installed.
  Package time-daemon is not installed.
  Package systemd-timesyncd which provides time-daemon is not installed.
  Package chrony which provides time-daemon is to be removed.

(Reading database ... 31279 files and directories currently installed.)
Removing chrony (3.5-6ubuntu5) ...
postrm called with unknown argument `remove'
dpkg: error processing package chrony (--remove):
 installed chrony package post-removal script subprocess returned error exit status 1
dpkg: too many errors, stopping
Errors were encountered while processing:
 chrony
Processing was halted because there were too many errors.
E: Sub-process /usr/bin/dpkg returned an error code (1)
root@ff-chrony:~# history
    1 apt install chrony
    2 apt install systemd-timesyncd
    3 history
root@ff-chrony:~# dpkg -l chrony
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-==============-============-============-=====================================================
rH chrony 3.5-6ubuntu5 amd64 Versatile implementation of the Network Time Protocol