annotate_delete_mailbox: don't nuke the per-mailbox file

With UUID paths, the per-mailbox annotations DB will be in the same
place, so when we "delete" the old entries, we don't want to remove
the file. This could have moved up into the !is_rename block, but
also this file gets cleaned up anyway, so we don't need to delete
it in annotate.c

Signed-off-by: Bron Gondwana <brong@fastmail.fm>