Fix an error of type ingore

[pingzh]

Run mypy..........................................................................................................................Failed
- hook id: mypy
- exit code: 1

airflow/models/dagrun.py:72: error: unused 'type: ignore' comment
            primaryjoin=and_(TI.dag_id == dag_id, TI.execution_date == exe...
            ^
Found 1 error in 1 file (checked 2408 source files)

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