ARM: S3C64XX: Remove un-used code backlight code on SMDK6410

According to commit 96d78686("ARM: S3C64XX: Add PWM backlight
support on SMDK6410") and commit f00207b2("ARM: SAMSUNG: Create
a common infrastructure for PWM backlight support"), this should
not be used anymore.

And this patch fixes follwing warning:
arch/arm/mach-s3c64xx/mach-smdk6410.c:296: warning: 'smdk6410_backlight_device' defined but not used

Signed-off-by: Banajit Goswami <banajit.g@samsung.com>
[kgene.kim@samsung.com: modified commit message]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>