Remove src/rxgk

Remove src/rxgk

These files were commited slightly prematurely to the tree; rxgk
support is intended for the 2.0 release, and will not appear in the
1.8.x release series.

Remove src/rxgk and drop mentions of rxgk from configure/Makefile.in/etc.

Change-Id: Ib7d40eaac85b05d920781b61f73dbdf8fedfcc2b
Reviewed-on: https://gerrit.openafs.org/12681
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>