switchdev: Remove unused transaction item queue

There are no more in tree users of the
switchdev_trans_item_{dequeue,enqueue} or switchdev_trans_item structure
in the kernel since commit 00fc0c51 ("rocker: Change world_ops API
and implementation to be switchdev independant").

Remove this unused code and update the documentation accordingly since.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>