net: pcs: xpcs: delete shim definition for mdio_xpcs_get_ops()

CONFIG_STMMAC_ETH selects CONFIG_PCS_XPCS, so there should be no
situation where the shim should be needed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>