chore(django-spanner): disable savepoints and remove legacy can_rollback_tests

Author: sinhasubham

packages/django-google-spanner/django_spanner/features.py

@@ -56,8 +56,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     # Spanner does not support SELECTing an arbitrary expression that also
     # appears in the GROUP BY clause.
     supports_subqueries_in_group_by = False
-    uses_savepoints = True
-    can_rollback_tests = False  # Spanner savepoints are no-ops; rely on flush.
+    uses_savepoints = False
     supports_aggregate_filter_clause = False
     # Spanner does not support expression indexes
     # example: CREATE INDEX index_name ON table (LOWER(column_name))