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:
<?php
// Implements hook_container().
use Drupal\Core\Template\Attribute;
function vatheme_container($variables) {
$request = \Drupal::request();
if ($route = $request->attributes->get(\Symfony\Cmf\Component\Routing\RouteObjectInterface::ROUTE_OBJECT)) {
$title = \Drupal::service('title_resolver')->getTitle($request, $route);
}
// Special handling for form elements.
$element = $variables['element'];
// Ensure #attributes is set.
$element += array('#attributes' => array());
if (isset($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 $element['#children'];
}
else{
return '<div' . new Attribute($element['#attributes']) . '>' . $element['#children'] . '</div>';
}
}
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.