Revert "board: rockchip: Add early ADC button detect for RGxx3"

This reverts commit 41a60d0e.

On some of the supported devices the adc detect code always returns
that the button has been pushed, and as a result the device will
not boot normally.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>