platform/x86: dell-laptop: Drop superfluous exported function

Since we've switched to the LED trigger for binding with HD-audio,
we can drop the exported function as well as the whole
linux/dell-led.h.

Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>