drivers: led_strip: ws2812: Remove WS2812_STRIP and WS2812_STRIP_DRIVER

Removing `choice WS2812_STRIP_DRIVER` to enable the use of multiple
types of WS2812 drivers.
Also, `menuconfig WS2812_STRIP` will be deleted as it does not
correspond to the appropriate settings.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>