repo-updater: remove updateScheduler mutex

Created by: keegancsmith

As far as I can tell this mutex serves no purpose. The only possible purpose is to serialize calls to UpdateFromDiff. However, the individual upsert and remove calls on the scheduler and queue have there own mutexes.

Possibly if we had two concurrent UpdateFromDiff we could have an inconsistent state, but it would converge at the next UpdateFromDiff call. Additionally we only have one call site for UpdateFromDiff, and that never runs concurrently.