Stop using the "undef" keyword to remove a constant

The keyword tries to remove a method of the same name which is unlikely
to be the intention of this test:

    $ ruby -e 'undef Object'
    -e:1:in '<main>': undefined method 'Object' for class 'Object' (NameError)

Found looking at GH-11497. The NameError triggers error_highlight, which
loads a bunch of file under GC.stress set by this test when using Prism.
That takes a long time, causing a timeout.

test/ruby/test_refinement.rb

@@ -825,7 +825,7 @@ class TestRefinement < Test::Unit::TestCase
       GC.stress = true
       10.times{
         #{PrependAfterRefine_CODE}
-        undef PrependAfterRefine
+        Object.send(:remove_const, :PrependAfterRefine)
       }
     }, timeout: 60
   end