nfsclient: Delete unused function nfscl_getcookie()

The function nfscl_getcookie(), which is essentially the
same as ncl_getcookie(), is never called, so delete it.
This is probably cruft left over from the port of the
NFSv4 code to FreeBSD several years ago.

Found while modifying the code to better use the
directory offset cookies.

MFC after: 2 weeks