Do not allow completely removing tag from all shards if currently in use (in config.tags collection)

If you remove a tag from all shards, but it still exists for a chunk (config.tags collection) then the chunk can not be migrated and causes errors during balancing, until the tag is added to a shard.

When removing a tag from a shard we should validate that it is not used by any chunks (config.tags collection) and if it is then it must not be the last shard with that tag.