nfsd: using nfsd4_encode_noop for encoding destroy_session/free_stateid

Get rid of the extra code, using nfsd4_encode_noop for encoding destroy_session and free_stateid.
And, delete unused argument (fr_status) int nfsd4_free_stateid.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>