net: dsa: mv88e6131: drop VLAN Ethertype setup

The 6131 switch models have a Core Tag Type register. Their setup code
is setting it to 0x8100, which is the reset default.

Drop this specific part which is correctly configured on reset anyway.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>