hw: silent warning about unknown signal context

Triggering of an invalidated signal seems to be no real exception,
but something that occurs regularily. Therefore, the kernel warning
is of no use to developers anymore.

Ref #3277

repos/base-hw/src/core/kernel/thread.cc

@@ -515,7 +515,7 @@ void Thread::_call_submit_signal()
 	/* lookup signal context */
 	Signal_context * const c = pd().cap_tree().find<Signal_context>(user_arg_1());
 	if(!c) {
-		Genode::raw(*this, ": cannot submit unknown signal context");
+		/* cannot submit unknown signal context */
 		user_arg_0(-1);
 		return;
 	}