R1:d3d41324be9f

Drop old "differential_commit" table

Summary: Ref T13276. Ref T13513. All readers and writers were removed more than a year ago; clean up the last remnants of this table.

Test Plan: Grepped for table references, found none.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13513, T13276

Differential Revision: https://secure.phabricator.com/D21281