NFS: Remove the left function defines in callback.h

Commit 778be232 "NFS do not find client in NFSv4 pg_authenticate" has remove
the define and using of nfs4_set_callback_sessionid(), and
commit 36281caa "NFSv4: Further clean-ups of delegation stateid validation"
has update the checking of stateid, and move the code to nfs4proc.c.

This patch remove those function defines left in callback.h

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>