Remove SlowvoteComment and storage

Summary: Ref T1191. This was migrated to transactions a very long time ago.

Test Plan: Ran migration, grepped, left comments in Slowvote.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10523