test: remove erroneous assert message from test

Removes the incorrect 'exit successfully' message from test when the
exit code is 0.

PR-URL: https://github.com/nodejs/node/pull/14918
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-process-external-stdio-close-spawn.js

@@ -22,7 +22,7 @@ if (process.argv[2] === 'child') {
   });
 
   child.on('close', common.mustCall((exitCode, signal) => {
-    assert.strictEqual(exitCode, 0, 'exit successfully');
+    assert.strictEqual(exitCode, 0);
     assert.strictEqual(signal, null);
   }));