net: dsa: remove legacy_pre_march2020 from drivers

Since DSA no longer marks anything as phylink-legacy, there is now no
need for DSA drivers to set this member to false. Remove all instances
of this.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>