Documentation: networking: dsa: remove references to switchdev prepare/commit

After the recent series containing commit bae33f2b ("net: switchdev:
remove the transaction structure from port attributes"), there aren't
prepare/commit transactional phases anymore in most of the switchdev
objects/attributes, and as a result, there aren't any in the DSA driver
API either. So remove this piece.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>