Revise comment about form submission without submit button data from FormBuilder

Problem/Motivation

FormBuilder::doBuildForm() contains specific support for an Internet Explorer quirk:

      // If a form contains a single textfield, and the ENTER key is pressed
      // within it, Internet Explorer submits the form with no POST data
      // identifying any submit button. Other browsers submit POST data as
      // though the user clicked the first button. Therefore, to be as
      // consistent as we can be across browsers, if no 'triggering_element' has
      // been identified yet, default it to the first button.
      $buttons = $form_state->getButtons();
      if (!$form_state->isProgrammed() && !$form_state->getTriggeringElement() && !empty($buttons)) {
        $form_state->setTriggeringElement($buttons[0]);
      }

It seems likely that this is obsolete now.

Steps to reproduce

Proposed resolution

GET forms (such as Views exposed filters) don't have to include a submit button input, so I think we should always default to the first button if we didn't find one - this makes sense to me and prevents breaking existing behaviour.

Therefore, the only thing to do here is update the comment so it's more accurate.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet