phy: phy-mtk-tphy: remove unused u3phya_ref clock

The u3phya_ref clock is already moved into sub-node, and
renamed as ref clock, no used anymore now, so remove it,
this can avoid confusion when support new platforms

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>