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