Remove unreachable return statement in AphrontFormDateControlValue
Summary: Two unconditional return statements lead to never reaching the second one.
Test Plan: Read the code of this method.
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D25729