As discovered by andreas.braun as part of CDRIVER-3552, the logic for checking whether inheriting a write concern is permissible in mongoc_collection_find_and_modify_with_opts is confusing.

1. Write concern is not inherited from the collection if it is unacknowledged.

Being unacknowledged should not prevent it from being inherited. It's possible the call to mongoc_write_concern_is_acknowledged was mistaken for mongoc_write_concern_is_default and that the intent was not to inherit the default write concern.

2. An acknowledged write concern is inherited from the collection and applied to an operation within a transaction

But transactions should not be inheriting write concerns from the collection.