ipq40xx: fix GL.iNet GL-B1300

The deletion of the mdio node childs was meant for testing and were
committed accidentally. Without the mdio nodes the network isn't
initialised.

While at it, remove the orphaned qcom-ipq4019-gl-b1300.dts as well.

Fixes: FS#1439
Signed-off-by: Mathias Kresin <dev@kresin.me>