phy: core: clean up unused ethernet specific phy modes

After recent changes PHY_MODE_SGMII, PHY_MODE_2500SGMII, PHY_MODE_QSGMII,
PHY_MODE_10GKR are not used any more and can be removed. Hence - remove
them.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>