Drop section moves when the section was deleted in IGListBatchUpdateData

[rnystrom]

⚠️ Blocked by #418 ⚠️

Whenever a batch update comes in with a delete and a move-from in the same section, we convert the move to a delete+insert. This is probably wrong b/c the data source count will likely change (b/c of the delete) but since we gather deletes in an NSIndexSet, only one delete will occur along w/ an insert from the move conversion.

https://github.com/Instagram/IGListKit/blob/master/Source/Common/IGListBatchUpdateData.mm#L92-L93

So pseudocode illustrating the problem:

let moves: Set<Move> = [Move(from: 0, to: 1)]
let deletes: IndexSet = IndexSet(index: 0)
let batch = IGListBatchUpdate(... moves: moves, deletes: deletes ...)
print(batch.moves) // empty
print(batch.deletes) // 0
print(batch.inserts) // 1

Instead, we should drop the move entirely so only the delete wins. That way the expected data source changes are reflected w/ the batch update.

In theory everything is working fine now b/c this situation wont exist in IGListDiff(...). However let's future-proof ourselves