specs: remove unused wait-for-ajax helper

This helper is:
- no longer used;
- buggy (not all requests increment it);
- discouraged (we should instead match an UI change that signals the end
of an ajax request).

Good riddance.

spec/support/wait_for_ajax.rb

@@ -1,9 +0,0 @@
-module WaitForAjax
-  def wait_for_ajax
-    expect(page).to have_selector('body[data-active-requests-count="0"]')
-  end
-end
-
-RSpec.configure do |config|
-  config.include WaitForAjax, type: :feature
-end