[8u] RFR: backport of JDK-8210886: Remove references in xwindows.md to non-existent files.

[Simon Tooke]

I would like to request a backport of  JDK-8210886: Remove references in
xwindows.md to non-existent files.

In JDK8, all the copyright notices are in THIRD_PARTY_README, which is
identical in all copies in the subrepos.

My patch only includes the base repo, but if it makes more sense, I can
create (identical) patches for all the subrepos.

Bug: https://bugs.openjdk.java.net/browse/JDK-8210886

Webrev: http://cr.openjdk.java.net/~stooke/webrevs/jdk-8210886/

Thank you for your time,

-Simon