grails clean deletes the exploded war directory as well

Since version 1.1.1 https://jira.grails.org/browse/GRAILS-3443 it's possible to keep the exploded WAR directory as the final artifact after `grails war`. But unfortunately there is not way to delete this exploded war directory using `grails clean`, which could possibly lead to deploying already deleted files.

This pull request adds the missing functionality to `grails clean`.