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
Add new line after create action on CommentsController code example
Merge pull request #12379 from francisgo/patch-7

Add new line after create action on CommentsController code example [ci skip]
Getting Started Guide: Update sentence in Chapter 5.8 [ci-skip]
Merge pull request #12233 from claudiob/delete-actionpack-unused-fixture

Remove HelperyTestHelper not used in any test
Merge pull request #12366 from francisgo/patch-4

Getting Started Guide: Update sentence in Chapter 5.8 [ci-skip]