Remove the UNGLOBALISE-related code

Since phpBB 3.1, the global announcements have been located in a given forum, just like any other topic type. When a normal topic (or a sticky or a regular announcement) turns into a global announcement, it remains in its original forum.

Oddly, Some codes related to UNGLOBALISE are still present (in phpBB 3.0, when you converted a global announcement to another type, we had to define a target forum). These codes are no longer useful since phpBB 3.1 and should be removed.

In phpBB 3.1.10 and 3.2.0 packages, some codes related to UNGLOBALISE still exist in posting.php, language/en/posting.php and styles/prosilver/template/posting_layout.html. In subSilver2 package for phpBB 3.1.10, a code related to UNGLOBALISE still exist in template/posting_body.html.