Disable repository read/write synchronization for now

Summary:
This nearly works but I didn't have time to get back to it and it isn't stable enough to turn on in the cluster yet.

We have enough other stuff going out this week, so just disable it before stable gets cut. Should be ready by next week if things go well.

Test Plan: Fetched a Git SSH repo locally.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15731