Rip out (currently not working) sync_status() and related test.

Summary:
Right now the failing test makes it harder to make sure ongoing work isn't
breaking anything new. I'm not really sure what's going on with the failing
test (it gets a timeout trying to query the sync server, is something
blocking?), but given that we're replacing this code with a new method of
keeping track of and querying sync status, I don't think it's worth our time to
debug it.

Test Plan: ./runtests

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D136