clang6: REMOVE misc/xnc

[Brian Callahan]

Hi ports --

misc/xnc has a copyright date of 96-98, hasn't seen an update since 
2001, has a dead HOMEPAGE, and now segfaults when I try to run it after 
fixing the clang6 build. Does anyone use this? Can we just send it off 
to the attic?

OK?

~Brian