core: hook up timer unit type with clean operation

core: hook up timer unit type with clean operation

timer units maintain state on disk (the persistent touch file), hence
let's expose cleaning it up generically with the new cleaning operation
for units.

This is a much simpler implementation as for the service unit type:
instead of forking out a worker process we just remove the touch file
directly. That should be OK since we only need to remove a single
(empty) file, instead of a recursive user-controlled directory tree.

Fixes: #4930