packet: remove deprecated syststamp timestamp

No device driver will ever return an skb_shared_info structure with
syststamp non-zero, so remove the branch that tests for this and
optionally marks the packet timestamp as TP_STATUS_TS_SYS_HARDWARE.

Do not remove the definition TP_STATUS_TS_SYS_HARDWARE, as processes
may refer to it.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>