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

Documentation/networking/driver.txt

@@ -58,13 +58,10 @@ Transmit path guidelines:
             TX_BUFFS_AVAIL(dp) > 0)
 		netif_wake_queue(dp->dev);
 
-2) Do not forget to update netdev->trans_start to jiffies after
-   each new tx packet is given to the hardware.
-
-3) An ndo_start_xmit method must not modify the shared parts of a
+2) An ndo_start_xmit method must not modify the shared parts of a
    cloned SKB.
 
-4) Do not forget that once you return 0 from your ndo_start_xmit
+3) Do not forget that once you return 0 from your ndo_start_xmit
    method, it is your driver's responsibility to free up the SKB
    and in some finite amount of time.