Revert "Handle uninitialized constant JSON::GenericObject at ruby/ruby."

This reverts commit 0dc1cd4.

1213adf is fixed this issue.

Author: hsbt

test/json/json_generic_object_test.rb

@@ -15,8 +15,6 @@ def setup
     else
       omit("JSON::GenericObject is not available")
     end
-  rescue NameError
-    omit("JSON::GenericObject is not available")
   end
 
   def test_attributes