pull xerces from central

When getting the project to build in my environment (I have a Nexus catchall proxy) I noticed a few repositories that do not seem to be needed anymore.

Historically xerces has been [a bit of a headache](http://stackoverflow.com/questions/11677572/dealing-with-xerces-hell-in-java-maven), but since Feb 13 it has been in central.  This means shibboleth-public repo is no longer needed.

The servicemix.m2-repo was unused so that has been removed too.