The form element type you're probably working with is a 'text_format' type. This is a composite element/widget. You can modify this form structure using `hook_form_alter()`. It's a bit hidden, but you can change 2 properties on the widget to control that dropdown. 

Make sure you are editing the form structure of the widget, not any containers. See the below code example for an idea on which element to update:

 $form['description']['widget'][0]['#format'] = 'full_html';
 $form['description']['widget'][0]['#allowed_formats'] = ['full_html'];

You can change 'full_html' to 'restricted_html' and see the editor change. Whatever mode you wish to enforce, you can set the machine name there and it'll set that as the default + hide the dropdown. 

See `\Drupal\filter\Element\TextFormat.php` for some more info (contained in the annotation at the top of the file):

 /**
 * Provides a text format render element.
 *
 * Properties:
 * - #base_type: The form element #type to use for the 'value' element.
 * 'textarea' by default.
 * - #format: (optional) The text format ID to preselect. If omitted, the
 * default format for the current user will be used.
 * - #allowed_formats: (optional) An array of text format IDs that are available
 * for this element. If omitted, all text formats that the current user has
 * access to will be allowed.
 *
 * Usage Example:
 * @code
 * $form['body'] = array(
 * '#type' => 'text_format',
 * '#title' => 'Body',
 * '#format' => 'full_html',
 * '#default_value' => '<p>The quick brown fox jumped over the lazy dog.</p>',
 * );
 * @endcode
 *
 * @see \Drupal\Core\Render\Element\Textarea
 *
 * @RenderElement("text_format")
 */