doc, net: Remove instruction to set net_device::trans_start

Commit 08baf561

 ('net:
txq_trans_update() helper') made it unnecessary for most drivers to
set net_device::trans_start (or netdev_queue::trans_start).

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>