Remove the last reference to event_txn_id. (#16521)

This table was no longer used, except for a background process
which purged old entries in it.

changelog.d/16521.misc

@@ -0,0 +1 @@
+Stop deleting from an unused table.

synapse/storage/databases/main/events_worker.py

@@ -2095,12 +2095,6 @@ class EventsWorkerStore(SQLBaseStore):
 
         def _cleanup_old_transaction_ids_txn(txn: LoggingTransaction) -> None:
             one_day_ago = self._clock.time_msec() - 24 * 60 * 60 * 1000
-            sql = """
-                DELETE FROM event_txn_id
-                WHERE inserted_ts < ?
-            """
-            txn.execute(sql, (one_day_ago,))
-
             sql = """
                 DELETE FROM event_txn_id_device_id
                 WHERE inserted_ts < ?

synapse/storage/schema/__init__.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-SCHEMA_VERSION = 82  # remember to update the list below when updating
+SCHEMA_VERSION = 83  # remember to update the list below when updating
 """Represents the expectations made by the codebase about the database schema
 
 This should be incremented whenever the codebase changes its requirements on the
@@ -121,6 +121,9 @@ Changes in SCHEMA_VERSION = 81
 Changes in SCHEMA_VERSION = 82
     - The insertion_events, insertion_event_extremities, insertion_event_edges, and
       batch_events tables are no longer purged in preparation for their removal.
+
+Changes in SCHEMA_VERSION = 83
+    - The event_txn_id is no longer used.
 """