rb_clear_method_cache_by_class: no longer exist

It is questionable whether we can change a public API but this function
does not exist today.  No way.

include/ruby/internal/intern/vm.h

@@ -52,7 +52,6 @@ void rb_define_alloc_func(VALUE, rb_alloc_func_t);
 void rb_undef_alloc_func(VALUE);
 rb_alloc_func_t rb_get_alloc_func(VALUE);
 void rb_clear_constant_cache(void);
-void rb_clear_method_cache_by_class(VALUE);
 void rb_alias(VALUE, ID, ID);
 void rb_attr(VALUE,ID,int,int,int);
 int rb_method_boundp(VALUE, ID, int);