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: #7599
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

Author: Trott

test/parallel/test-child-process-fork.js

@@ -7,12 +7,9 @@ var args = ['foo', 'bar'];
 var n = fork(common.fixturesDir + '/child-process-spawn-node.js', args);
 assert.deepStrictEqual(args, ['foo', 'bar']);
 
-var messageCount = 0;
-
 n.on('message', function(m) {
   console.log('PARENT got message:', m);
   assert.ok(m.foo);
-  messageCount++;
 });
 
 // https://github.com/joyent/node/issues/2355 - JSON.stringify(undefined)