nfsd: clean up comments over nfs4_client definition

nfsd fault injection has been deprecated since
commit 9d60d931 ("Deprecate nfsd fault injection")
and removed by
commit e56dc9e2

 ("nfsd: remove fault injection code")

So remove the outdated parts about fault injection.

Signed-off-by: Chen Hanxiao <chenhx.fnst@fujitsu.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>