Remove HelperyTestHelper not used in any test

HelperyTestHelper was introduced in 66ef922 by @josevalim
to pair with HelperyTestController. This test controller was
later removed in e10a253 by @strzalek, leaving HelperyTestHelper unused
Merge pull request #12233 from claudiob/delete-actionpack-unused-fixture

Remove HelperyTestHelper not used in any test