test: remove unused var from child-process-fork

`messageCount` is assigned, but never used. Remove it.

(This was missed by the linter in previous versions of ESLint but is
flagged by the current version. Updating the linter is contingent on
this change or some similar remedy landing.)

PR-URL: https://github.com/nodejs/node/pull/7599


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>