backlight: Remove pxa tosa support

The PXA tosa machine was removed, so this backlight driver is no
longer needed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230105134622.254560-10-arnd@kernel.org