Remove hart_form_builder from assets

[chrisma]

Helper class is in assets folder: app/assets/helpers/hart_form_builder.rb.
This means that Rails asset pipeline will attempt to compile and serve the file.

[bdaase]

Where should it be instead?

[bdaase]

Btw. it was placed here, because we followed the instructions from https://coderwall.com/p/-biuug/custom-form-builders-in-rails.

[chrisma]

See #486