Fix deletion of multipaste tarball when deleting a multipaste

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Properly delete multipastes when deleting a single paste

mmultipaste->delete_id() also deletes multipaste tarballs which would
otherwise be left over

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Simplify mfile->delete_hash

This probably increases the database queries a bit, but greatly
simplifies the code and fixes the missing deletion of multipaste
tarballs (delete_id handles that).

Signed-off-by: Florian Pritz <bluewind@xinu.at>