Removing forced JUnit dep.

As http://stackoverflow.com/questions/7898446/how-to-exclude-a-dependency-from-parents-project-in-maven notes, it is impossible to _remove_ a dependency specified in a parent POM. Generally a POM project just should not declare dependencies at all; it can use `dependencyManagement` where useful.

Affects `jenkins-test-harness`, which is trying to depend on JUnit 4.10 but is also stuck with this obsolete JUnit 3.8 from a parent.