Change: Make `unlock` display message only when locks were actually removed

The `unlock` command used to print the "successfully removed locks" message
whenever it was run, regardless of lock files having being removed or not.

This has now been changed such that it only prints the message if any lock
files were actually removed. In addition, it also reports the number of
removed lock files.

https://github.com/restic/restic/issues/3929
https://github.com/restic/restic/pull/3935
