etherdevice: remove unused eth_addr_greater

eth_addr_greater() was introduced for the mv88e6xxx driver, but is not
used anymore. There is no other user, thus remove this function.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>