remove rb_obj_iv_index_tbl

(1) nobody uses it (gem-codesearch)
(2) the data strucuture will be changed.

include/ruby/internal/core/robject.h

@@ -61,10 +61,6 @@ struct RObject {
     } as;
 };
 
-RBIMPL_SYMBOL_EXPORT_BEGIN()
-struct st_table *rb_obj_iv_index_tbl(const struct RObject *obj);
-RBIMPL_SYMBOL_EXPORT_END()
-
 RBIMPL_ATTR_PURE_UNLESS_DEBUG()
 RBIMPL_ATTR_ARTIFICIAL()
 static inline uint32_t
@@ -97,17 +93,4 @@ ROBJECT_IVPTR(VALUE obj)
     }
 }
 
-RBIMPL_ATTR_DEPRECATED(("Whoever have used it before?  Just tell us so.  We can stop deleting it."))
-RBIMPL_ATTR_PURE_UNLESS_DEBUG()
-RBIMPL_ATTR_ARTIFICIAL()
-static inline struct st_table *
-ROBJECT_IV_INDEX_TBL(VALUE obj)
-{
-    RBIMPL_ASSERT_TYPE(obj, RUBY_T_OBJECT);
-
-    struct RObject *const ptr = ROBJECT(obj);
-
-    return rb_obj_iv_index_tbl(ptr);
-}
-
 #endif /* RBIMPL_ROBJECT_H */

object.c

@@ -320,14 +320,6 @@ rb_obj_singleton_class(VALUE obj)
     return rb_singleton_class(obj);
 }
 
-struct st_table *
-rb_obj_iv_index_tbl(const struct RObject *obj)
-{
-    /* This is a function that practically never gets used.  Just to keep
-     * backwards compatibility to ruby 2.x. */
-    return ROBJECT_IV_INDEX_TBL((VALUE)obj);
-}
-
 /*! \private */
 MJIT_FUNC_EXPORTED void
 rb_obj_copy_ivar(VALUE dest, VALUE obj)