Dart forms page: remove reference to FORM_DIRECTIVES

[chalin]

This is a follow-up to #2282. What is needed is to remove these lines from public/docs/dart/latest/guide/forms.jade:

.l-sub-section
  :marked
    Note the `platform_directives` entry in `pubspec.yaml`.
    It imports core directives and, more importantly for this chapter,
    **form directives**.