In the following code, the iclass tree of refinements in cref should be <iclass of Kernel@M2> -> <iclass of Kernel@M1> -> Kernel. However, the iclass tree was broken because of code for included modules of refinements in rb_using_refinement(). Refinement#include is now removed, so this commit removes such unnecessary code. ```ruby module M1 refine(Kernel) do def f1 = :f1 end end module M2 refine(Kernel) do def f2 = :f2 end end class Foo using M1 using M2 def test p f2 #=> :f2 p f1 # expected => :f1 # actual => undefined local variable or method 'f1' for an instance of Foo end end Foo.new.test ```