Clean up a duplicated definition

I simply forgot deleting it.

test/irb/test_cmd.rb

@@ -511,7 +511,7 @@ def test_show_source
 
     def test_show_source_end_finder
       pend if RUBY_ENGINE == 'truffleruby'
-      code = <<~EOS.gsub(/^/, ' ' * 4); eval(code, binding, __FILE__, __LINE__ + 1) unless respond_to?(:show_source_test_method)
+      eval(code = <<-EOS, binding, __FILE__, __LINE__ + 1)
         def show_source_test_method
           unless true
           end
@@ -549,11 +549,5 @@ def test_whereami
       assert_empty err
       assert_match(/^From: .+ @ line \d+ :\n/, out)
     end
-
-    # This method is used for testing show_source command
-    def show_source_test_method
-      unless true
-      end
-    end
   end
 end