platform/x86: thinkpad_acpi: 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/thinkpad_acpi.h.

The own TPACPI_LED_MUTE and TPACPI_LED_MICMUTE definitions are
replaced with the identical ones for LEDS, i.e. LED_AUDIO_MUTE and
LED_AUDIO_MICMUTE, respectively.  They are no longer needed as
referred only locally.

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: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>