docs/templates/helpers.rb: remove unused code
docs/templates/helpers.rb: remove unused code

In the previous commit, we imported helpers.rb, as well as other
template files, from the asciidoctor-backends[1] repo.

However, as we did not import all of the template files, some of the
code in helpers.rb is unused.

The unused code is an unnecessary maintenance and readability burden, so
let's remove them.

[1] https://github.com/asciidoctor/asciidoctor-backends/tree/master/slim/html5

se-edu/addressbook-level3