[ios] Removes mismatched function declaration. (#15423)

platform/ios/src/MGLMapView_Private.h

@@ -52,7 +52,6 @@ FOUNDATION_EXTERN MGL_EXPORT MGLExceptionName const _Nonnull MGLUnderlyingMapUna
 /// Synchronously render a frame of the map.
 - (void)renderSync;
 
-- (nonnull mbgl::Map *)mbglMap;
 - (nonnull mbgl::Renderer *)renderer;
 
 /** Returns whether the map view is currently loading or processing any assets required to render the map */