Remove "backout", "close", "flag", "start", "stop", "time", and "revert" workflows

Summary: Ref T13490. These workflows don't seem worth the maintenance cost, see T13488 for discussion.

Test Plan: Grepped for methods which looked like they might only be called by these flows, only dug up the backout stuff.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21100