metal: don't try to autogenerate depth mips, that isn't supported

src/modules/graphics/metal/Graphics.mm

@@ -958,10 +958,6 @@ void Graphics::endPass()
 		if (rt.texture->getMipmapsMode() == Texture::MIPMAPS_AUTO && rt.mipmap == 0)
 			rt.texture->generateMipmaps();
 	}
-
-	int dsmipmap = rts.depthStencil.mipmap;
-	if (depthstencil != nullptr && depthstencil->getMipmapsMode() == Texture::MIPMAPS_AUTO && dsmipmap == 0)
-		depthstencil->generateMipmaps();
 }
 
 void Graphics::clear(OptionalColorf c, OptionalInt stencil, OptionalDouble depth)