pwm-backlight: Remove unused variable

I forgot to remove this during earlier cleanup patches and only checked
various builds for errors, not warnings.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/video/backlight/pwm_bl.c

@@ -45,7 +45,7 @@ struct pwm_bl_data {
 
 static void pwm_backlight_power_on(struct pwm_bl_data *pb, int brightness)
 {
-	int duty_cycle, err;
+	int err;
 
 	if (pb->enabled)
 		return;