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`. This may cause that already deleted files eventually being deployed, because they never get deleted in the exploded war directory.

This pull request adds the missing functionality to `grails clean`.
 
IMHO it's not very likely that it is used. I added this target for completeness and because I used it sometimes during development in order to just delete the exploded war directory and keep the other compiled artifacts. It's just a minor improvement for the build process and not necessarily required.