R1:a19f49632fd8

Remove willWriteRevision/didWriteRevision hooks

Summary:
Ref T2222. DifferentialRevisionEditor has no remaining callsites, but it has a bit of functionality which still needs to be ported forward. I'm going to rip it apart piece by piece.

This removes the willWriteRevision/didWriteRevision hooks. They are completely encapsulated by transactions now, except for a unique piece of branch/task logic, which I migrated forward.

Test Plan:
- Lots of `grep`.
- Created a new revision on branch `T25`, saw it…