ARM: dts: ux500: Remove ab8500_ldo_usb regulator from device tree

Support for the USB regulator of AB8500 was removed in
commit 41a06aa7 ("regulator: ab8500: Remove USB regulator").
However, the configuration was never removed from the device tree.

It does no longer have any effect, remove it from the device tree.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>