Deorbit the netscape plugin

Netscape hasn't been a thing since 2008.

We would only try to build it if someone manually ran 'webinstall'
in src/libuafs, and there is no documentation telling anyone to do so.

It's dead, Jim.

Change-Id: I7486ca33da7ff19f23f0d9f54f5fb4e7e3232540
Reviewed-on: http://gerrit.openafs.org/11470
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>