libceph: remove req->r_replay_version

Nothing uses this anymore with the removal of the ack vs. commit code.
Remove the field and just encode zeroes into place in the request
encoding.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>