Exclude the extraneous xmlpull and xpp3_min dependencies in jenkins-core

The xpp3_min artifact causes a "JBAS015893: Encountered invalid class name" error on deployment in JBoss AS7 due to a malformed manifest in the artifact.

When I researched further I discovered that the `xpp3_min` artifact is extraneous since the `xpp3` library is a direct dependency for jenkins-core. The xpp3 library also replaces the `xmlpull` library.

jenkins-core passed all unit tests. Installed a WAR into my local system and I haven't encountered any issues.