Problem with "remove scheduled OBs" checkbox feature

[ejeschke]

@monodera reports:
By unchecking the 'remove scheduled OBs' box with the current 'HSC-S17A' files, I got the following error message at scheduling from qplan.

2017-03-31 12:54:12,296 | E | Task.py:387 (execute) | Task '<qplan.main.QueuePlanner.main.<locals>.QueuePlanner object at 0x11a181e48>.task3' terminated with exception: list.remove(x): x not in list
2017-03-31 12:54:12,334 | E | Task.py:391 (execute) | Traceback:
  File "/Users/monodera/anaconda3/envs/qplan/lib/python3.5/site-packages/ginga/misc/Task.py", line 377, in execute
    res = self.func(*self.args, **self.kwdargs)
  File "/Users/monodera/anaconda3/envs/qplan/lib/python3.5/site-packages/qplan-2.6.3.dev1362-py3.5.egg/qplan/Scheduler.py", line 453, in schedule_all
    props[str(ob.program)].obs.remove(ob)
2017-03-31 12:54:12,334 | E | Task.py:952 (execute) | Task '<qplan.main.QueuePlanner.main.<locals>.QueuePlanner object at 0x11a181e48>.task3' raised exception: list.remove(x): x not in list