test: remove flaky designation

Searching the automatically-generated test failure reports in the issues
at https://github.com/nodejs/reliability/issues, I don't see
`test-worker-message-port-message-before-close` failing any time
recently.

PR-URL: #51736
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

test/parallel/parallel.status

@@ -55,8 +55,6 @@ test-domain-with-abort-on-uncaught-exception: PASS, FLAKY
 test-tick-processor-arguments: SKIP
 
 [$system==freebsd]
-# https://github.com/nodejs/node/issues/31280
-test-worker-message-port-message-before-close: PASS,FLAKY
 
 [$system==ibmi]
 # https://github.com/nodejs/node/pull/30819