platform/x86: fujitsu-laptop: remove backlight-related attributes from the platform device

Setting backlight level using a vendor-specific interface should only be
possible when using the latter is either explicitly requested by the
user or automatically selected by the kernel.  fujitsu-laptop violates
that premise by unconditionally attaching three backlight-related
attributes to the platform device it registers.  Remove the offending
attributes from the platform device.  Update module comments to reflect
this change.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Tested-by: Jonathan Woithe <jwoithe@just42.net>
Reviewed-by: Jonathan Woithe <jwoithe@just42.net>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>