nfsd: eliminate unused nfs4_callback.cb_stat

The cb_stat member of struct nfs4_callback is unused
since commit ff7d9756

 nfsd: use static memory for callback program and stats

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>