Starting a discussion after setting MWPS removes the MWPS status

As an example, this MR wasn't merged automatically once its pipeline succeeded, despite its Merge When Pipeline Succeeds status: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29160

Screen_Shot_2019-06-04_at_6.04.57_PM

The MWPS status was removed silently and is not reported in the system notes.

/cc @ClemMakesApps