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