Remove unused type comment that upsets mypy (#10877)

Something slightly odd is happening here.

In #10729 it passed the mypy tests with this in place, but now if I make
a change to this file (or to scheduler_job.py, which imports DagRun) I
get an error _with_ this.

GitOrigin-RevId: 5cc8e9826bef303723b82ee28f020a26506dac7b

airflow/models/dagrun.py

@@ -69,7 +69,7 @@ class DagRun(Base, LoggingMixin):
 
     task_instances = relationship(
         TI,
-        primaryjoin=and_(TI.dag_id == dag_id, TI.execution_date == execution_date),  # type: ignore
+        primaryjoin=and_(TI.dag_id == dag_id, TI.execution_date == execution_date),
         foreign_keys=(dag_id, execution_date),
         backref=backref('dag_run', uselist=False),
     )