<AWT Dev> RFR: 8210866: Remove HPKeysym.h from JDK sources

[Phil Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8210880
Webrev : http://cr.openjdk.java.net/~prr/8210866/

As described in the bug, we can rely on the build platform for this file,
as we do for most X11 include files. Builds pass on our core platforms.
Since this is an X11 protocol file, I expect very few people will find they
are missing this and have to install it ..

-phil.