While not really documented, it is generally assumed that:
* the fieldset is a user-visible string (much like a title),
* it will be translated by whichever piece of code uses it.

So:
* fix AutomaticEntityForm to generate untranslated fieldset names,
* fix FormRenderer to properly translate the fieldset name before
  rendering it,
* stop using the field set (which may contain whitespace) as a CSS class
  name (the feature was unusable, so just drop it),
* properly handle the colon as part of the translated fieldset name

Closes #4881298