nfs: remove unnecessary NFS_NEED_* defines

Thanks to Robert Day for pointing out that these two defines are unused.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Trond Myklebust <trond@netapp.com>Trond Myklebust <trond@netapp.com>
Cc: Neil Brown <neilb@suse.de>
Cc: "Robert P. J. Day" <rpjday@crashcourse.ca>