getrusage(2): zero out ru_nsignals field

The current value was both wrong (counting spawned kernel signals
rather than delivered user signals) and returned for the calling
process even if the request was for the process's children.
For now we are better off not populating this field at all.

Change-Id: I6c660be266b5746b7c3db57ae88fa7f872961ee2

minix/kernel/proc.c

@@ -1983,8 +1983,3 @@ void ser_dump_proc()
                 print_proc_recursive(pp);
         }
 }
-
-void increase_proc_signals(struct proc *p)
-{
-	p->p_signal_received++;
-}