test: remove flaky status for cluster test

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

Remove it's flaky status.

Fixes: #2557
PR-URL: #3975
Reviewed-By: Rod Vagg <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Roman Klauke <[email protected]>

Author: Trott

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