Remove stale comment about TI index (#39470)

The index this comment is talking about was removed in #30762.

airflow/models/taskinstance.py

@@ -1387,14 +1387,6 @@ class TaskInstance(Base, LoggingMixin):
         Index("ti_dag_run", dag_id, run_id),
         Index("ti_state", state),
         Index("ti_state_lkp", dag_id, task_id, run_id, state),
-        # The below index has been added to improve performance on postgres setups with tens of millions of
-        # taskinstance rows. Aim is to improve the below query (it can be used to find the last successful
-        # execution date of a task instance):
-        #    SELECT start_date FROM task_instance WHERE dag_id = 'xx' AND task_id = 'yy' AND state = 'success'
-        #    ORDER BY start_date DESC NULLS LAST LIMIT 1;
-        # Existing "ti_state_lkp" is not enough for such query when this table has millions of rows, since
-        # rows have to be fetched in order to retrieve the start_date column. With this index, INDEX ONLY SCAN
-        # is performed and that query runs within milliseconds.
         Index("ti_pool", pool, state, priority_weight),
         Index("ti_job_id", job_id),
         Index("ti_trigger_id", trigger_id),