Administrators and members of projects with specific permissions should be able to delete comments in Maniphest.
Use case: vandals, spammers, users accidentally leaking sensitive information.
Deleting would mean removing the information. A reference to the deleted post would be left in the UI, but there shouldn't be any way to access to the removed content by accessing to previous versions or diffs.
If deleting means making them private and accessible only to users with permissions to //delete// comments, that might work as well.