test: remove watchdog in test-debug-signal-cluster

test-debug-signal-cluster contains a watchdog timer that results in
false positives in CI. Remove the watchdog timer and let the test runner
determine that the test has timed out.

PR-URL: nodejs#9476
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-debug-signal-cluster.js

@@ -65,10 +65,6 @@ function onNoMoreDebuggerAgentsOutput() {
   process.exit();
 }
 
-setTimeout(function testTimedOut() {
-  common.fail('test timed out');
-}, common.platformTimeout(4000)).unref();
-
 process.on('exit', function onExit() {
   // Kill processes in reverse order to avoid timing problems on Windows where
   // the parent process is killed before the children.