Remove the (hopefully) obsolete "post_max_size" check during startup

Summary:
Ref T13507. See that task for discussion. This check appears to be obsolete in all common cases and misfires if the client submits compressed requests.

Since the cases where it could still trigger correctly are extremely rare and should still have plausible behavior, just remove it.

Test Plan: Grepped for calls.

Maniphest Tasks: T13507

Differential Revision: https://secure.phabricator.com/D21077