disable automatic failed card removal

It has caused two issues in the last few days:

- A user thought their failed cards were mysteriously disappearing
- A few users with 1 day+ re-learning steps had thought they hit a bug. What happens is they fail a card on day 1 and its review interval is set to day 2; then a few days later after a few steps, they fail it again and it enters queue 1, but when the queue is rebuilt the steps are discarded because the original review repetition is due.

It might be possible to fix the above, but for 2.0 it seems like the safest bet is to remove this feature, especially as nobody actually asked for it and I just thought it might be handy.

anki/sched.py

@@ -23,7 +23,6 @@ class Scheduler(object):
         self.reportLimit = 1000
         self.reps = 0
         self._haveQueues = False
-        self._clearOverdue = True
         self._updateCutoff()
 
     def getCard(self):
@@ -40,8 +39,6 @@ class Scheduler(object):
     def reset(self):
         deck = self.col.decks.current()
         self._updateCutoff()
-        if self._clearOverdue:
-            self.removeFailed(expiredOnly=True)
         self._resetLrn()
         self._resetRev()
         self._resetNew()
@@ -191,8 +188,6 @@ order by due""" % self._deckLimit(),
     def deckDueList(self):
         "Returns [deckname, did, rev, lrn, new]"
         self._checkDay()
-        if self._clearOverdue:
-            self.removeFailed(expiredOnly=True)
         self.col.decks.recoverOrphans()
         decks = self.col.decks.all()
         decks.sort(key=itemgetter('name'))