Remove unused AV/test/fixtures/happy_path

The `test/fixtures/happy_path/render_action/hello_world.erb` file was
introduced in 8ab37c7 for the `TestRenderAction` test.

That test was subsequently removed in 34f058e, so the fixture is not
used anymore.

If Travis CI is happy, then you can be sure the fixture can be removed.
Merge pull request #17877 from claudiob/remove-happy-path-fixture

Remove unused AV/test/fixtures/happy_path