fix: Revert "Feat: Threaded MutationsBatcher" (#773)

Reverts #722

This PR caused beam bigtableio.py failures https://togithub.com/apache/beam/issues/26673 and is blocking beam release. We're unclear why it caused the failure. So will revert this change, cut another release so we can unblock beam and investigate separately.

Author: mutianf

docs/batcher.rst

@@ -17,7 +17,6 @@ Using the API
   row-data
   row-filters
   row-set
-  batcher
 
 
 In the hierarchy of API concepts