nfsd: Remove nfsd_luid, nfsd_lgid, nfsd_ruid and nfsd_rgid

These trivial macros that don't currently do anything are the last
vestiages of an old attempt at uid mapping that was removed from the
kernel in September of 2002.  Remove them to make it clear what the
code is currently doing.

Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>