[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
```

1 files changed, 29 insertions, 0 deletions

diff --git a/test/ruby/test_refinement.rb b/test/ruby/test_refinement.rb
index 826616eb09..cb55627e2c 100644
--- a/test/ruby/test_refinement.rb
+++ b/test/ruby/test_refinement.rb
@@ -2672,6 +2672,35 @@ class TestRefinement < Test::Unit::TestCase
     assert_equal(:v2, obj.cached_foo_callsite)
   end
 
+  # [Bug #20302]
+  def test_multiple_refinements_for_same_module
+    assert_in_out_err([], <<-INPUT, %w(:f2 :f1), [])
+      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
+          p f1
+        end
+      end
+
+      Foo.new.test
+    INPUT
+  end
+
   private
 
   def eval_using(mod, s)