Change robot deletions to set the performer to null, rather than attempting to delete the rows from the large log entries table

test/test_api_usage.py

@@ -1965,6 +1965,9 @@ class TestOrgRobots(ApiTestCase):
     pull_robot = model.get_user(membername)
     model.create_build_trigger(repo, 'fakeservice', 'sometoken', user, pull_robot=pull_robot)
 
+    # Add some log entries for the robot.
+    model.log_action('pull_repo', ORGANIZATION, performer=pull_robot, repository=repo)
+
     # Delete the robot and verify it works.
     self.deleteResponse(OrgRobot,
                         params=dict(orgname=ORGANIZATION, robot_shortname='bender'))