Omit unnecessary function call

Summary: As mentioned on rP8ce35e6b67e7e2a81b274bab7a6dd19dedb4df06, setConcreteOnly(true) can be omitted since (lacking magical powers) loadObjects() will always only instantiate concrete objects.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10050