RFR (JDK10/JAXP Doc-only) 8196717: remove xmlresolver.md

[Joe Wang]

Hi,

Refer to JDK-8177561[1], XML Commons Resolver was removed from JDK 10. 
xmlresolver.md therefore needs to be removed as well.

Please review:
https://bugs.openjdk.java.net/browse/JDK-8196717
http://cr.openjdk.java.net/~joehw/jdk10/8196717/webrev/

[1] https://bugs.openjdk.java.net/browse/JDK-8177561

Thanks,
Joe