extcon: Remove duplicated include from extcon-class.c

Remove duplicated "of.h" header file.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

drivers/extcon/extcon-class.c

@@ -32,7 +32,6 @@
 #include <linux/of.h>
 #include <linux/slab.h>
 #include <linux/sysfs.h>
-#include <linux/of.h>
 
 /*
  * extcon_cable_name suggests the standard cable names for commonly used