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.

Author: lpinca

test/parallel/parallel.status

@@ -61,8 +61,6 @@ test-tick-processor-arguments: SKIP
 [$system==freebsd]
 # https://github.com/nodejs/node/issues/31727
 test-fs-stat-bigint: PASS,FLAKY
-# 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