testCMFActivity: do not rely on unittest internals (_resultForDoCleanups)

_resultForDoCleanups no longer exists on python3, getting the status
of a test in teardown is very different accross python versions ( see
https://stackoverflow.com/q/4414234 ) and just executing this code when
the test failed only has the problem that a test method may count as
two failures or errors, which is after all not a big problem.

product/CMFActivity/tests/testCMFActivity.py

@@ -138,7 +138,6 @@ class TestCMFActivity(ERP5TypeTestCase, LogInterceptor):
     # activity messages. We are testing CMFActivity so it's important to check
     # that everything works as expected on this subject.
     try:
-      if self._resultForDoCleanups.wasSuccessful():
       getMessageList = self.portal.portal_activities.getMessageList
       self.assertFalse(getMessageList())
       # Also check if a test drop them without committing.