docs: net: dsa: remove port_bridge_tx_fwd_offload

We've changed the API through which we can offload the bridge TX
forwarding process. Update the documentation in light of the removal of
2 DSA switch ops.

Fixes: b079922b ("net: dsa: add a "tx_fwd_offload" argument to ->port_bridge_join")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>