[ticket/11864] Do not call exit after display_progress_bar in acp_forums

The progress bar is called with the method display_progress_bar() in the
acp_forums files. This method then call adm_page_footer() which also calls
exit_handler(). Therefore, the exit; in acp_forums after
$this->display_progress_bar() is obsolete and shouldn't exist in the first
place.

PHPBB3-11864