How do I remove  from output?
Okay, I figured it out. After setting the services.yml file debug to "true" and enabling comments in my Firefox console, I was able to get the theme output for the particular set of divs that was adding the unwanted markup (I tried this earlier, but I think I forgot to rename default.services.yml to services.yml).

Anyway, I found that the markup was coming from Bootstrap's container.html.TWIG template. Of course, this is one of the templates that doesn't come with override suggestions, just the hook theme_container() theme function. So I copied the default function into my [subtheme name].theme file at the top of my subtheme's directory and modified it thus to suit my needs:

&lt;?php
// Implements hook_container().
use Drupal\Core\Template\Attribute;

function vatheme_container($variables) {

  $request = \Drupal::request();

  if ($route = $request-&gt;attributes-&gt;get(\Symfony\Cmf\Component\Routing\RouteObjectInterface::ROUTE_OBJECT)) {
    $title = \Drupal::service('title_resolver')-&gt;getTitle($request, $route);
  }

// Special handling for form elements.
$element = $variables['element'];
 // Ensure #attributes is set.
$element += array('#attributes' =&gt; array());

        if (isset($element['#array_parents'])) {
            // Assign an html ID.

            if (!isset($element['#attributes']['id'])) {
                $element['#attributes']['id'] = $element['#id'];
            }

            // Add the 'form-wrapper' class.
            $element['#attributes']['class'][] = 'form-wrapper';
        }

        if($title == 'Contact Tech Support'){           
            return $element['#children']; 
        }
        else{
            return '&lt;div' . new Attribute($element['#attributes']) . '&gt;' . $element['#children'] . '&lt;/div&gt;';
        }
}


So, that seems to do the trick. This was my first stab at doing theme work in Drupal 8 and there are a lot of not insignificant changes from Drupal 7 here, so I thought I'd go through the details of my solution. Hopefully this helps someone else out there.