Remove not used attribute _schema_file

This was actually a remaining of [1].

[1]: https://review.openstack.org/#/c/115918

Change-Id: Idcf589fa9d89c2969914ef0c8aee27a22325470e
diff --git a/tempest/test.py b/tempest/test.py
index 14cf3bb..7db0376 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -414,12 +414,8 @@
         else:
             standard_tests, module, loader = args
         for test in testtools.iterate_tests(standard_tests):
-            schema_file = getattr(test, '_schema_file', None)
             schema = getattr(test, '_schema', None)
-            if schema_file is not None:
-                setattr(test, 'scenarios',
-                        NegativeAutoTest.generate_scenario(schema_file))
-            elif schema is not None:
+            if schema is not None:
                 setattr(test, 'scenarios',
                         NegativeAutoTest.generate_scenario(schema))
         return testscenarios.load_tests_apply_scenarios(*args)