leds: flash: Remove synchronized flash strobe feature

Synchronized flash strobe feature has been considered not fitting
for LED subsystem sysfs interface and thus is being removed.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>