Clean iOS platform config of long gone macro

(cherry picked from commit 8c2a577350924d1f4a8e0a98ebf85bea96ea7e63)

platform/iphone/platform_config.h

@@ -33,8 +33,6 @@
 #define GLES2_INCLUDE_H <ES2/gl.h>
 #define GLES3_INCLUDE_H <ES3/gl.h>
 
-#define PLATFORM_REFCOUNT
-
 #define PTHREAD_RENAME_SELF
 
 #define _weakify(var) __weak typeof(var) GDWeak_##var = var;