R1:48f6189f3226

Allow mailing lists to be permanently deleted

Summary: Allow `PhabricatorMetaMTAMailingList` to be permanently deleted with `./bin/remove destroy`.

Test Plan:
```
./bin/remove destroy PHID-MLST-nseux3r55escj573shsf
IMPORTANT: OBJECTS WILL BE PERMANENTLY DESTROYED!

There is no way to undo this operation or ever retrieve this data.

These 1 object(s) will be completely destroyed forever:

- PHID-MLST-nseux3r55escj573shsf (PhabricatorMetaMTAMailingList)

Are you absolutely certain you want to…