Revert a vestige of mock

tests/unit/test__opentelemetry_tracing.py

@@ -30,14 +30,7 @@ def _make_rpc_error(error_cls, trailing_metadata=None):
 def _make_session():
     from google.cloud.spanner_v1.session import Session
 
-    session = mock.Mock(autospec=Session, instance=True)
-    # Setting _observability_options to None is to avoid the nasty spill-over
-    # of mock._tracer_provider spuriously failing tests, because per
-    # unittest.mock.Mock's definition invoking any attribute or method
-    # returns another mock.
-    db = session._database
-    setattr(db, "observability_options", None)
-    return session
+    return mock.Mock(autospec=Session, instance=True)
 
 
 # Skip all of these tests if we don't have OpenTelemetry