Support combine.self="remove"

[mwisnicki]

Ability to remove inherited nodes would be super useful for maven: https://issues.apache.org/jira/browse/MNG-6808

Spec:
https://oss.sonatype.org/service/local/repositories/releases/archive/org/atteo/xml-combiner/2.2/xml-combiner-2.2-javadoc.jar/!/org/atteo/xmlcombiner/CombineSelf.html#REMOVE

[rfscholte]

Fixed with #75