Made `+[MGLAccountManager setMapboxMetricsEnabledSettingShownInApp:]` unavailable, with a message explaining what to do instead. Removed a commented-out call to that method. Only the environment for an access token if one hasn’t already been set in Info.plist.
That way there’s no ambiguity about when you should call things like `+[MGLAccountManager setMapboxMetricsEnabledSettingShownInApp:]`. In fact, that method is now deprecated because it’s so easy to call in the wrong place.
Fixes#1535.