nfsd: discard nfserr_dropit

nfserr_dropit hasn't been used for over a decade, since rq_dropme and
the RQ_DROPME were introduced.

Time to get rid of it completely.

Signed-off-by: NeilBrown <neil@brown.name>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>