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