test: remove mark flaky for moved test

Remove 'flaky' in parallel.status for test-debug-signal-cluster as the
test was moved to sequential.

Refs: https://github.com/nodejs/node/pull/13592
PR-URL: https://github.com/nodejs/node/pull/19069


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>