test_have_macro.rb: unused variables

* test/mkmf/test_have_macro.rb (test_have_macro_header),
  (test_not_have_macro_header): remove unused variables.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38891 b2dd03c8-39d4-4d8f-98ff-823fe69b080e