docs: net: dsa: delete port_mdb_dump

This was deleted in 2017, stop documenting it.

Fixes: dc0cbff3 ("net: dsa: Remove redundant MDB dump support")
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>