fix: remove flaky test_staff_response test

The test test_staff_response failed and then passed
and subsequent runs with the same code, and is now
considered flaky. It is being removed in accordance with
this flaky test process: https://2u-internal.atlassian.net/wiki/spaces/TE/pages/12812492/Flaky+Test+Process
Note: the Flaky+Test+Process doc should probably be more
public, but simply notes that these tests should be removed
with a ticket to allow the owning team to fix it.

Here is a link to the job where it failed and passed: 
https://github.com/openedx/edx-platform/actions/runs/3752596945/jobs/6375603154

lms/djangoapps/discussion/rest_api/tests/test_api.py

@@ -4099,14 +4099,6 @@ def test_default_response(self):
         topics_list = get_course_topics_v2(course_key=self.course_key, user=self.user)
         assert {t['id'] for t in topics_list} == set(self.topic_ids)
 
-    def test_staff_response(self):
-        """
-        Test that the standard response contains the correct number of items
-        """
-        topics_list = get_course_topics_v2(course_key=self.course_key, user=self.staff)
-        # All topic ids should be returned except for the first deleted topic id which has zero stats
-        assert {t['id'] for t in topics_list} == self.all_topic_ids - {self.deleted_topic_ids[0]}
-
     def test_filtering(self):
         """
         Tests that filtering by topic id works