Deregister csv renderer after test to prevent leak.

ActionController does not seem to provide an API to remove renderers. Therefore we need to undo the actions in `ActionController::Renderers#add`. Otherwise, it will lead to the failure of `test_raises_missing_renderer_if_an_api_behavior_with_no_renderer` if `test_uses_renderer_if_an_api_behavior` is run first.

cc @senny