repo-updater: remove streaming syncer feature flag
Created by: keegancsmith
It has defaulted to on for many releases.
Additionally I noticed a bug. If SyncedSubset is nil we should not do streaming inserts. The reason is Synced will never have the Diff.Added field populated since it will always be via subset synced. So this would lead to the repo not being noticed in the git update scheduler. However, we always have SubsetSynced set (except in tests). So this was a benign bug.