tests/common.h: Remove unused prototype

tests/common.h

@@ -27,7 +27,6 @@
 
 extern void reset(cJSON *item);
 extern char *read_file(const char *filename);
-extern cjbool assert_is_invalid(cJSON *item);
 
 /* assertion helper macros */
 #define assert_has_type(item, item_type) TEST_ASSERT_BITS_MESSAGE(0xFF, item_type, item->type, "Item doesn't have expected type.")