net: dsa: mt7530: remove the .port_set_mrouter implementation

DSA's idea of optimizing out multicast flooding to the CPU port leaves
quite a few holes open, so it should be reverted.

The mt7530 driver is the only new driver which added a .port_set_mrouter
implementation after the reorg from commit a8b659e7 ("net: dsa: act
as passthrough for bridge port flags"), so it needs to be reverted
separately so that the other revert commit can go a bit further down the
git history.

Fixes: 5a30833b ("net: dsa: mt7530: support MDB and bridge flag operations")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>