Improve BigPipe's use of ajax.js

I mean if we're there why not be even more direct:

var commands = new Drupal.AjaxCommands();
var dummyAjax = {getEffect: Drupal.Ajax.prototype.getEffect}; 
response.forEach(function (item) {
  if (commands[item.command]) {
    commands[item.command](dummyAjax, item);
  }
});

And we should get rid of the hard requirement on getEffect in core so we don't need the dummy function.

I'd rather not have people use Drupal.Ajax directly, and nobody knows what the parameters of Drupal.Ajax() are so having false, false doesn't help make it clearer. Whereas using Drupal.ajax you can do

Drupal.ajax({
  url: '',
  // The rest is done by default but you can be extra explicit for clarity.
  base: false,
  element: false,
  progress: false,
});

All good :)