Remove calls to DifferentialRevision::loadRelations from commit workers

Summary:
Remove ocurrences of `loadRelations` in workers.

One was simply unnecesary, no subsequent call to `getReviewers` or `getCCPHIDs` was made.
The other was replaced with the nicer `DifferentialRevisionQuery` using `needRelations` and `needReviewerStatus` (for future upgrade).

Test Plan:
Land a revision into a tracked repository and check the parser worker attached the commit correctly.
For the owners worker I just checked it didn't crash into a hundred tiny pieces.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6467