Remove dead code in `tests/storage/test_database.py` (#12197)

Signed-off-by: Sean Quah <seanq@element.io>

changelog.d/12197.misc

@@ -0,0 +1 @@
+Remove some dead code.

tests/storage/test_database.py

@@ -13,26 +13,10 @@
 # limitations under the License.
 
 from synapse.storage.database import make_tuple_comparison_clause
-from synapse.storage.engines import BaseDatabaseEngine
 
 from tests import unittest
 
 
-def _stub_db_engine(**kwargs) -> BaseDatabaseEngine:
-    # returns a DatabaseEngine, circumventing the abc mechanism
-    # any kwargs are set as attributes on the class before instantiating it
-    t = type(
-        "TestBaseDatabaseEngine",
-        (BaseDatabaseEngine,),
-        dict(BaseDatabaseEngine.__dict__),
-    )
-    # defeat the abc mechanism
-    t.__abstractmethods__ = set()
-    for k, v in kwargs.items():
-        setattr(t, k, v)
-    return t(None, None)
-
-
 class TupleComparisonClauseTestCase(unittest.TestCase):
     def test_native_tuple_comparison(self):
         clause, args = make_tuple_comparison_clause([("a", 1), ("b", 2)])