[Bug #20302] Multiple refinements cannot be applied to the same module

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
```

0 files changed, 0 insertions, 0 deletions