nfsd4: remove spkm3

Unfortunately, spkm3 never got very far; while interoperability with one
other implementation was demonstrated at some point, problems were found
with the spec that were deemed not worth fixing.

The kernel code is useless on its own without nfs-utils patches which
were never merged into nfs-utils, and were only ever available from
citi.umich.edu.  They appear not to have been updated since 2005.

Therefore it seems safe to assume that this code has no users, and never
will.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>