Revert unread optimisation, has too many edge cases

spec/models/user_stat_spec.rb

@@ -10,34 +10,6 @@ describe UserStat do
     expect(user.user_stat.new_since).to be_present
   end
 
-  context "#update_first_topic_unread_at" do
-    it "updates date correctly for staff" do
-      now = Time.zone.now
-
-      admin = Fabricate(:admin)
-      topic = Fabricate(:topic,
-                          highest_staff_post_number: 7,
-                          highest_post_number: 1,
-                          last_unread_at: now
-                       )
-
-      UserStat.update_first_topic_unread_at!
-
-      admin.reload
-
-      expect(admin.user_stat.first_topic_unread_at).to_not be_within(5.years).of(now)
-
-      TopicUser.change(admin.id, topic.id, last_read_post_number: 1,
-                                           notification_level: NotificationLevels.all[:tracking])
-
-      UserStat.update_first_topic_unread_at!
-
-      admin.reload
-
-      expect(admin.user_stat.first_topic_unread_at).to be_within(1.second).of(now)
-    end
-  end
-
   context '#update_view_counts' do
 
     let(:user) { Fabricate(:user) }