NFS: Clean up nfs4_xdr_dec_secinfo()

Clean up: Remove superfluous logic at the tail of
nfs4_xdr_dec_secinfo() .

Introduced by commit 5a5ea0d4 "NFS: Add secinfo procedure" (March 24,
2011).

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>