test: remove erroneous assert message from test

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

PR-URL: #14918
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

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);
   }));