Remove unused import Case from dagrun.py (#52179)

Author: gopidesupavan

airflow-core/src/airflow/models/dagrun.py

@@ -58,7 +58,6 @@
 from sqlalchemy.ext.hybrid import hybrid_property
 from sqlalchemy.ext.mutable import MutableDict
 from sqlalchemy.orm import declared_attr, joinedload, relationship, synonym, validates
-from sqlalchemy.sql.elements import Case
 from sqlalchemy.sql.expression import false, select
 from sqlalchemy.sql.functions import coalesce
 from sqlalchemy_utils import UUIDType