leds: remove config option LEDS_GPIO_PLATFORM from Kconfig

Since the commit a314c5c0
(leds/leds-gpio: merge platform_driver with of_platform_driver),
the config option LEDS_GPIO_PLATFORM becomes useless, so remove it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
[grant.likely: also remove LEDS_GPIO_OF for same reason]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>