Xt/Xaw interface removed

[Stan Shebs]

As discussed last week, I removed the old Xt/Xaw interface.  The
final state of the code is available via the CVS tag
"xtconq-pre-removal", although I didn't confirm that it built.
The 7.3 version is known to work OK, and is what I plan to use
when comparing behavior.

So now we need to get the networking code really working well,
there is no fallback for multiplayer!

Stan