Revert "signals: Return NULL at SigThread end."

This reverts commit 3c55f6d8.

This is dead code (which does generate a warning!!!)

modules/control/signals.c

@@ -118,7 +118,4 @@ static void *SigThread (void *data)
     pthread_sigmask (SIG_UNBLOCK, &set, NULL);
     for (;;)
         pause ();
-
-    /* Not reached */
-    return NULL;
 }