Currently if a shard is in draining mode but is the only shard in a particular zone, the balancer will just skip this chunk and the command will never return "completed". If removeShard is run with the the forceRemoveByAbandoningData option, we should fail the command in this case and inform the user they must add a new shard to this zone. We cannot add a shard to this zone automatically due to various restrictions (i.e. GDPR), and it is unacceptable in this case for the shard to remain in forcedDraining mode.