Remove deprecated `/members/mailgun/permanent_failures` route
In !86226 (merged) (15.0), we moved this to /mailgun/webhooks
so that it also handles other mailgun events.
We can remove this legacy route in the next major release (16.0)
This feature was mainly built for GitLab.com's Mailgun integration so there would probably be very few instances using this. Removing this route is still a breaking change though.