Fix crash when deleting a task and its descendant together

[gycsaba96]

When deleting a task and one of its descendants simultaneously, the program crashes. It happens because it tries to remove the descendant twice: once as a selected task and once as the descendant of the other task.

Steps to reproduce the bug:

1. Start GTG with the default dataset.
2. Expand the first task ("Getting Started with GTG (read me first)").
3. Select the parent tag and its children.
4. Try to delete them using simultaneously.
5. Observe the crash.

When deleting multiple tasks simultaneously, we should check that a given task was not already removed as the descendant of a previously removed task.

[SqAtx]

I guess you didn't want to do the obvious

item = self.lookup.get(item_id)
if not item:
  return

probably because it might potentially hide problems instead of making us fix them?

I'm fine with that idea. LGTM - I'd just clarify the docstring.

[gycsaba96]

I guess you didn't want to do the obvious

item = self.lookup.get(item_id)
if not item:
  return

probably because it might potentially hide problems instead of making us fix them?

Exactly. :) Moreover, doing this would also require us to change the specification of BaseStore.remove, which could potentially break other parts of the code.

[diegogangl]

LGTM, thanks!