fix(tests): drop removed subtransactions= kwarg from Session.begin()

TestDatasource.setUp explicitly opened a transaction with
db.session.begin(subtransactions=True) before each test, relying on
tearDown's rollback() to isolate them. subtransactions was already
deprecated in SQLAlchemy 1.4 and is removed outright in 2.0
(TypeError: unexpected keyword argument 'subtransactions'), surfacing
as a failure while investigating discussion #40273's SQLAlchemy 2.0
bump.

The explicit begin() is unnecessary either way: Session autobegins on
first use under both 1.4 and 2.0, so tearDown's rollback() still
correctly discards whatever the test did without it.

tests/integration_tests/datasource_tests.py

@@ -107,9 +107,6 @@ def create_and_cleanup_table(table=None):
 
 
 class TestDatasource(SupersetTestCase):
-    def setUp(self):
-        db.session.begin(subtransactions=True)
-
     def tearDown(self):
         db.session.rollback()
         super().tearDown()