Fix source item removals in sorted derived cols

The test contained is kind of contrived and it could probably be
simplified a lot. Any way, this is a stupid bug and it's all on me. It's
not possible to use filter as the test for whether or not to include an
item when handling removes since the state of the item might have changed
since it was added (or not added).

I also used sourceIndex where I should have been using destinationIndex
which caused the wrong items to get removed.

Now it actually checks if the item is included (by going through the
mapping list instead).