Fix build, only ask for location permission on user's instigation

Previously in mapbox#2265 we would ask for location permission at app startup if `!settings->showsUserLocation`. mapbox#2193 0a172a2 changed the way we used `NSUserDefaults` and mostly removed the `settings` object, which broke mapbox#2265.

Rather than fix our pestering location permissions ask at startup, this commit now only asks for location permissions when the user hits the locate-me button.

Once a user grants permission, the user dot appears because a `userTrackingMode` is set, `showsUserLocation` is permanently set to `YES` in `NSUserDefaults` and is restored at launch.

ios/app/MBXViewController.mm

@@ -83,13 +83,6 @@ - (void)viewDidLoad
                                                                              action:@selector(locateUser)];
 
     [self restoreState:nil];
-
-    if ( ! settings->showsUserLocation)
-    {
-        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
-            self.mapView.showsUserLocation = YES;
-        });
-    }
 }
 
 - (void)saveState:(__unused NSNotification *)notification