JENKINS-17508 possible fix

As this is my first look inside Jenkins codebase I not sure it's correct but perhaps artifacts for modulebuilds should be deleted just as logs
are deleted. see https://issues.jenkins-ci.org/browse/JENKINS-17508
I did not fully test it yet, all unit tests ran fine, but due to classloader issues while running
Jenkins in Jetty I didn't succeed to run jobs inside Jenkins to verify deletion