Revert to consuming a single datacenter until the canary system is more robust.

In T338796 we had decided to depend on all datacenter pre-partitions so that we do not need to manually intervene every time a datacenter switchover happens. However, we have had multiple instances in which canary events fail to be generated and this has made the sensors for multiple pipelines brittle. Thus, reverting these changes until we have a more robust canary system.

Bug: T338796