-[MGLStyle removeLayer:] relies on undefined behavior

[1ec5]

Per #6097 (comment), the -removeFromMapView: method in each of the concrete MGLStyleLayer subclasses relies on undefined behavior by reinterpret_casting a unique_ptr to the mbgl::style::Layer. This code was added to the iOS and macOS SDKs in #7048. The Android SDK is doing something a bit different in #7039: move-ing a unique_ptr but not reinterpret_casting it.

/cc @jfirebaugh @boundsj

[boundsj]

This was fixed by #7227