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 file. This method then calls adm_page_footer() which also calls exit_handler(). Therefore, the exit; in acp_forums after $this->display_progress_bar is useless and shouldn't exist in the first place (should've always been exit_handler() + gargabe_collection()).