asus-laptop: remove deprecated interfaces (lcd_switch and display_get)

I should have done that one year ago, so it's more than
time to do it.

These two features use non-standard interfaces. There are the
only features that really need multiple path to guess what's
the right method name on a specific laptop.

Removing them allow to remove a lot of code an significantly
clean the driver.

This will affect the backlight code which won't be able to know
if the backlight is on or off.

The platform display file will also be write only (like the one
in eeepc-laptop).

Signed-off-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>