test_super.rb: remove unneeded code

* test/ruby/test_super.rb (test_missing_super_in_module_unbound_method):
  binding target does not need extending.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44552 b2dd03c8-39d4-4d8f-98ff-823fe69b080e