staging/lustre/ptlrpc: get rid of lustre 1.8 compatible checksums

Old version of lustre (1.8) calculated RPC checksums differently,
but the compat code is no longer important since in-kernel
version of lustre is nto compatible with 1.8 lustre servers
(that are long deprecated too).

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>