Currently, there is a helper function `enterStableTopologyRegion` which exposes the shard membership lock in shared mode. We do not want to encourage/allow its usage because it is error prone as it only guards the actual change to config.shards (see SERVER-112610). Places which need to serialize with add/remove shard should rather use the DDL lock on config.shards which serializes with the entire operation.

This ticket is to remove this helper. Currently, its only users are setUserWriteBlockMode and setClusterParameter, both of which have other, higher level synchronizations with add shard now (since the usage of the shard membership lock was once again incorrect there) and so they no longer need this lock.