Replace the remaining Node.removeChild() instances with Element.remove()

Using `Element.remove()` is a slightly more compact way of removing an element, since you no longer need to explicitly find/use its parent element.
Furthermore, the patch also replaces a couple of loops that're used to delete all elements under a node with simply overwriting the contents directly (a pattern already used throughout the viewer).

See also:
 - https://developer.mozilla.org/en-US/docs/Web/API/Node/removeChild
 - https://developer.mozilla.org/en-US/docs/Web/API/Element/remove

extensions/chromium/contentscript.js

@@ -128,7 +128,7 @@ function updateEmbedElement(elem) {
   var parentNode = elem.parentNode;
   var nextSibling = elem.nextSibling;
   if (parentNode) {
-    parentNode.removeChild(elem);
+    elem.remove();
   }
   elem.type = "text/html";
   elem.src = getEmbeddedViewerURL(elem.src);