Remove auto-indexing from MongoEngine

Summary:
BE restarts are starting to cause site issues by flooding thousands of
index_information() calls at once. We don't really use this feature for
much anymore and it's causing greif, so ripping it out.

Test Plan: Ran Wish unit tests and clicked around. Nothing was broken.

Reviewers: josh, jack, tarek, david

Differential Revision: http://phab.wish.com/D17974