Revert "swaybg: Make swaybg a shell surface"

This reverts commit 99bda4afe2.

It turned out that code to handle swaybg as shell surface was broken so we don't
want to make swaybg a shell surface until this has been fixed.

swaybg/main.c

@@ -54,7 +54,6 @@ int main(int argc, const char **argv) {
 		sway_abort("Failed to create surfaces.");
 	}
 	desktop_shell_set_background(registry->desktop_shell, output->output, window->surface);
-	window_make_shell(window);
 	list_add(surfaces, window);
 
 #ifdef WITH_GDK_PIXBUF