Ensure shards delete their TransactionCoordinator when restarting at the active transaction number

A shard may be chosen as the coordinator by mongos when it is executing a statement in a transaction, but if the targeting round fails, mongos will clear its tracking state and may pick a different shard as the coordinator when it retries. If a shard created a coordinator and then has its transaction restarted by the retry attempt (i.e. receives a request with startTransaction=true at the active transaction number), we should ensure its TransactionCoordinator object was removed.