rootwrap: remove redundant sanity check

now that we're checking the return value from `setuid(uid)` as of the
previous commit.

(see discussion of [MR 47])

[MR 47]: videolan/vlc!47)

bin/rootwrap.c

@@ -249,9 +249,6 @@ int main (int argc, char *argv[])
     if (setuid (uid))
         exit (1);
 
-    if (!setuid (0)) /* sanity check: we cannot get root back */
-        exit (1);
-
     /* Yeah, the user can execute just about anything from here.
      * But we've dropped privileges, so it does not matter. */
     if (strlen (argv[0]) < sizeof ("-wrapper"))