net: axienet: Unexport and remove unused mdio functions

Both axienet_mdio_{enable/disable} functions are no longer used in
xilinx_axienet_main.c due to 253761a0. And axienet_mdio_disable is
not even used in the mdio.c. So unexport and remove them.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>