cluster: remove debug arg handling

--debug and --debug-brk are no longer valid flags so remove special
handling for them in the cluster module. Even if they are restored, they
will be aliases for inspect and will not use the legacy debug protocol,
so the special handling will not be needed.

PR-URL: https://github.com/nodejs/node/pull/12738


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>