[CB-4556] Add a ./cordova/clean project-level script

https://issues.apache.org/jira/browse/CB-4556
@rodms10 could you check this file?
Our clean node script already outputs the same text, as you can see here: https://github.com/apache/cordova-firefoxos/blob/master/bin/templates/project/cordova/lib/clean.js#L36

You can remove this ECHO. Everything else looks good and works great. Thanks!