Remove flaky diff note tests for now
These will be replaced with better, more exhaustive specs by @lbennett based on the conversation in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5262.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
These will be replaced with better, more exhaustive specs by @lbennett based on the conversation in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5262.