Remove ReleephRequestEvent

Summary: Ref T3663. This is obsolete code which is used only in this migration, which Facebook has already performed and which isn't relevant for any other installs.

Test Plan: grep

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3663

Differential Revision: https://secure.phabricator.com/D6777