arm: dts: rockchip: rk3399-puma: remove u-boot,boot-led

arm: dts: rockchip: rk3399-puma: remove u-boot,boot-led

There's no code to make use of it.

Further more, the HW default state of that LED is on and migrating this
to the LED_BOOT implem brings no benefit as it'll stay on if U-Boot
reaches its main-loop. Blinking the LED_BOOT also doesn't help because
it doesn't blink for long enough to be noticeable before it's kept on.

This is by design, c.f.
https://source.denx.de/u-boot/u-boot/-/blob/v2025.10/include/led.h#L32-34

If we want this LED to be doing something different, it'll need to be
handled by a board file anyway.

Considering it hasn't worked in many years, let's just remove it.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>