LibWeb: Do not spin the event loop on processing iframe attributes

Partially reverting a3149c1ce9

Spinning the event loop was causing a crash on:

https://wpt.live/url/percent-encoding.window.html

As it was turning what is meant to be a synchronous operation into an
asynchronous one.

The sequence demonstrated by the reproducing test is as follows:
  * A src attribute is changed for the iframe
  * process_the_iframe_attributes entered with valid content navigable
  * Event loop is spun, allowing the queued iframe removal to execute
  * process_the_iframe_attributes continues with null content navigable
  * 💥

(cherry picked from commit 30377e6e3500049c5442fac9d242d3b795861116)

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp

@@ -86,12 +86,6 @@ void HTMLIFrameElement::process_the_iframe_attributes(bool initial_insertion)
     if (!content_navigable())
         return;
 
-    // Make sure applying of history step caused by potential sync navigation to "about:blank"
-    // is finished. Otherwise, it might interrupt navigation caused by changing src or srcdoc.
-    if (!initial_insertion && !content_navigable_initialized()) {
-        main_thread_event_loop().spin_processing_tasks_with_source_until(Task::Source::NavigationAndTraversal, [this] { return content_navigable_initialized(); });
-    }
-
     // 1. If element's srcdoc attribute is specified, then:
     if (has_attribute(HTML::AttributeNames::srcdoc)) {
         // 1. Set element's current navigation was lazy loaded boolean to false.