[fbs][2/n] Remove .python3 markers
Test Plan:
`xbgr '\.python3'` shows only one (dead) usage of this file:
https://www.internalfb.com/intern/diffusion/FBS/browse/master/fbcode/python/repo_stats/buck.py?commit=9a8dd3243207819325d520c208218f6ab69e4e49&lines=854
Reviewed By: lisroach
Differential Revision: D22955631
fbshipit-source-id: e686d9157c08c347d0ce4acdd05bd7ab29ff7df5
