nfs: Remove dead case from nfs4_map_errors()

NFS4ERR_ACCESS has number 13 and thus is matched and returned
immediately at the beginning of nfs4_map_errors() and there's no point
in checking it later.

Coverity-id: 733891
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>