test: remove flaky status for cluster test

The test did not fail after 9999 runs in continuous integration.

Remove it's flaky status.

Fixes: https://github.com/nodejs/node/issues/2557
PR-URL: https://github.com/nodejs/node/pull/3975
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>

test/parallel/parallel.status

@@ -10,7 +10,6 @@ prefix parallel
 test-tls-ticket-cluster           : PASS,FLAKY
 
 [$system==linux]
-test-cluster-worker-forced-exit   : PASS,FLAKY
 test-http-client-timeout-event    : PASS,FLAKY
 test-tls-no-sslv3                 : PASS,FLAKY
 test-child-process-buffering      : PASS,FLAKY