Remove Sponsor 55 label from open tickets when Sponsor 55 is closed

Once Sponsor 55 is closed out, we should remove the ~"Sponsor 55" label from remaining tickets that we didn't get to. For each ticket, we should decide what to do with it.

Is it a must-fix issue for 0.4.5 that follows from our S55 work? If so we should make sure it is assigned and scheduled and put in the 045 milestone, with an 045-must label if needed.

Is it a longstanding issue that we put under S55 because it logically fit with our work there? If so then we should either defer or reschedule it as appropriate.