libsnapshot: Cleanup iterators

The libsnapshot iterators can all use the same interface instead of
duplicating the interface. We don't have any need for the internal class
variable iterators, so remove them.

Test: Builds, does not impact cow_api_test/cow_snapuserd_test
Change-Id: I5f008401e067a55a57812b7bf101a472ad97df18