Remove unused admin/announcements helper (#23899)

app/helpers/admin/announcements_helper.rb

-# frozen_string_literal: true
-
-module Admin::AnnouncementsHelper
-  def time_range(announcement)
-    if announcement.all_day?
-      safe_join([l(announcement.starts_at.to_date), ' - ', l(announcement.ends_at.to_date)])
-    else
-      safe_join([l(announcement.starts_at), ' - ', l(announcement.ends_at)])
-    end
-  end
-end