platform: fix return value for do_delete_object()

The return value for the delete methods checks whether the object
is actually deleted. That is questionable behavior, because if the netlink
request succeeds, there is little point in checking with the platform cache.
As it is, it is racy.

Anyway, the previous value was totally wrong.

But it also uncovers another platform bug, which currently breaks
route tests. Will be fixed next.

src/platform/nm-linux-platform.c

@@ -4032,7 +4032,7 @@ out:
 	/* such an object still exists in the cache. To be sure, refetch it (and
 	 * hope it's gone) */
 	do_request_one_type (platform, NMP_OBJECT_GET_TYPE (obj_id));
-	return !!nmp_cache_lookup_obj (cache, obj_id);
+	return !nmp_cache_lookup_obj (cache, obj_id);
 }
 
 static WaitForNlResponseResult