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