Remove ReleephEvent

Summary:
Ref T3663. This is a proto-transaction record which is obsoleted by real transactions. It has no UI, so I'm not bothering to retain/migrate the data since there's no regression.

Just get rid of it and all its writers. I'm keeping the table for now in case something crazy uses this somehow, so no data is actually destroyed.

Test Plan: grep

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3663

Differential Revision: https://secure.phabricator.com/D6784