Remove all callers to "DifferentialRevision->loadIDsByCommitPHIDs()"

Summary: Depends on D20457. Ref T13276. Kill all remaining callers to this method and delete it.

Test Plan:

• Grepped for loadIDsByCommitPHIDs.
• Viewed blame again to make sure I didn't break it.
• Viewed "History" view for commits with revisions.
• Viewed "Graph" view for commits with revisions.
• Viewed "Merged Commits" table for commits with revisions.
• Viewed "Compare" table for commits with revisions.
• Viewed "Repository" main page history table for commits with revisions.
• Grepped for linkRevision.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13276

Differential Revision: https://secure.phabricator.com/D20458