how to remove WIDTH and HEIGHT from img-tag in Drupal 8
In your theme open file THEMENAME.theme, example in my case THEMENAME is bartik and file is bartik.theme add code below:

function bartik_preprocess_image(&amp;$variables) {
  if (isset($variables['attributes']['width']) &amp;&amp; isset($variables['attributes']['height'])) {
    unset($variables['attributes']['width']);
    unset($variables['attributes']['height']);
  }
}


Clear all cache and see result.