of: drop duplicate headers

Drop duplicate headers string.h and of_platform.h.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/overlay.c

@@ -18,7 +18,6 @@
 #include <linux/string.h>
 #include <linux/ctype.h>
 #include <linux/errno.h>
-#include <linux/string.h>
 #include <linux/slab.h>
 #include <linux/err.h>
 #include <linux/idr.h>

drivers/of/resolver.c

@@ -18,7 +18,6 @@
 #include <linux/string.h>
 #include <linux/ctype.h>
 #include <linux/errno.h>
-#include <linux/string.h>
 #include <linux/slab.h>
 
 /* illegal phandle value (set when unresolved) */

drivers/of/unittest.c

@@ -17,7 +17,6 @@
 #include <linux/slab.h>
 #include <linux/device.h>
 #include <linux/platform_device.h>
-#include <linux/of_platform.h>
 
 #include <linux/i2c.h>
 #include <linux/i2c-mux.h>