DERBY-6624: Use javax.xml.xpath interfaces for XPath support

Stop using the interfaces in the org.w3c.dom.xpath package, since they
are not part of the Java SE specification.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1604758 13f79535-47bb-0310-9956-ffa450edef68