backlight: remove ADX backlight device support

Support for the Avionic Design Xanthos backlight device got added in
commit 3b96ea9e ("backlight: Add support for the Avionic Design Xanthos
backlight device.").  That support depends on ARCH_PXA_ADX.  The code that
should have provided that Kconfig symbol never got submitted.  It has
never been possible to even build this driver.  Remove it.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Thierry Reding <thierry.reding@avionic-design.de>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>