dt-bindings: cros-ec: Remove google,cros-kbd-led-backlight

dt-bindings: cros-ec: Remove google,cros-kbd-led-backlight

After applying 4c03a44e2668 ("arm64: dts: mediatek: mt8195-cherry: Remove
keyboard-backlight node"), there are no users for using the OF match.
Instead, the device is added via drivers/mfd/cros_ec_dev.c by
970c3a6b7aa3 ("mfd: cros_ec: Register keyboard backlight subdevice").

Remove the DT bindings.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Acked-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20241206031405.1711996-3-tzungbi@kernel.org
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>