R1:580590fcc9fd

Remove Controller->getLoadedHandles()

Summary: Ref T7689. Removes this part of the `Controller->loadHandles()` + `Controller->getLoadedHandles()` mechanism.

Test Plan:
- Viewed Herald transcripts.
- Viewed Maniphest tasks with attached revisions and commits.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7689

Differential Revision: https://secure.phabricator.com/D12204