nfsd: remove unused stats counters

Commit 501cb184 ("nfsd: rip out the raparms cache") removed the
code that updates read-ahead cache stats counters,
commit 8bbfa9f3 ("knfsd: remove the nfsd thread busy histogram")
removed code that updates the thread busy stats counters back in 2009
and code that updated filehandle cache stats was removed back in 2002.

Remove the unused stats counters from nfsd_stats struct and print
hardcoded zeros in /proc/net/rpc/nfsd.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>