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>

Documentation/networking/dsa/dsa.rst

@@ -862,10 +862,6 @@ Bridge VLAN filtering
   the specified MAC address from the specified VLAN ID if it was mapped into
   this port forwarding database.
 
-- ``port_mdb_dump``: bridge layer function invoked with a switchdev callback
-  function that the driver has to call for each MAC address known to be behind
-  the given port. A switchdev object is used to carry the VID and MDB info.
-
 Link aggregation
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