Make _account_data_id_gen always an AbstractStreamIdGenerator to appe…

…ase mypy

As get_next_txn is part of AbstractStreamIdGenerator, not
AbstractStreamIdTracker. I'm not sure why we insisted in trying to type
it as a Tracker vs. a Generator, and then checking that elsewhere in the
codebase. The init function of AccountDataWorkerStore will always create
_account_data_id_gen as either a StreamIdGenerator or a
MultiWriterIdGenerator. Both of which are AbstractStreamidGenerators.

synapse/storage/databases/main/account_data.py

@@ -63,14 +63,12 @@ def __init__(
     ):
         super().__init__(database, db_conn, hs)
 
-        # `_can_write_to_account_data` indicates whether the current worker is allowed
-        # to write account data. A value of `True` implies that `_account_data_id_gen`
-        # is an `AbstractStreamIdGenerator` and not just a tracker.
-        self._account_data_id_gen: AbstractStreamIdTracker
         self._can_write_to_account_data = (
             self._instance_name in hs.config.worker.writers.account_data
         )
 
+        self._account_data_id_gen: AbstractStreamIdGenerator
+
         if isinstance(database.engine, PostgresEngine):
             self._account_data_id_gen = MultiWriterIdGenerator(
                 db_conn=db_conn,
@@ -475,7 +473,6 @@ async def add_account_data_to_room(
             The maximum stream ID.
         """
         assert self._can_write_to_account_data
-        assert isinstance(self._account_data_id_gen, AbstractStreamIdGenerator)
 
         content_json = json_encoder.encode(content)
 
@@ -514,7 +511,6 @@ async def remove_account_data_for_room(
             The maximum account data stream position.
         """
         assert self._can_write_to_account_data
-        assert isinstance(self._account_data_id_gen, AbstractStreamIdGenerator)
 
         def _remove_account_data_for_room_txn(txn: LoggingTransaction) -> Optional[int]:
             """
@@ -590,7 +586,6 @@ async def add_account_data_for_user(
             The maximum stream ID.
         """
         assert self._can_write_to_account_data
-        assert isinstance(self._account_data_id_gen, AbstractStreamIdGenerator)
 
         async with self._account_data_id_gen.get_next() as next_id:
             await self.db_pool.runInteraction(
@@ -696,7 +691,6 @@ async def remove_account_data_for_user(
             The maximum account data stream position.
         """
         assert self._can_write_to_account_data
-        assert isinstance(self._account_data_id_gen, AbstractStreamIdGenerator)
 
         def _remove_account_data_for_user_txn(txn: LoggingTransaction) -> Optional[int]:
             """