[PKTGEN]: Remove write-only variable.

The pktgen_thread.pid is set to current->pid and is never used
after this. So remove this at all.

Found during isolating the explicit pid/tgid usage.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>