Remove .sql3 hacks from Differential

Summary:
See T178. D372 is the correct fix for this problem, hardcoding .sql3 is not.

Test Plan:
This code should be unreachable after T178 since these files will always be
marked as binary.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen
CC: elgenie, aran, tuomaspelkonen
Differential Revision: 373