Remove batch_insert_unconfirmed from TxGraph and IndexedTxGraph

[LLFourn]

We have:

• batch_insert_unconfirmed
• batch_insert_relevant_unconfirmed
• batch_insert_relevant

We only need batch_insert_relevant which should have the API:

    pub fn batch_insert_relevant<'t>(
        &mut self,
        txs: impl IntoIterator<Item = &'t Transaction>,
    ) -> ChangeSet<A, I::ChangeSet> {

Whether they are confirmed or unconfirmed (have only anchors or last seen) is not important for doing this job. The caller can add anchors and last seen with the methods available for that. Also the documentation should make it clear why this method exists in the first place: because relevancy cannot be determined until all of the transactions have been first scanned by the indexer.

Also there is a batch_insert_unconfirmed in TxGraph that doesn't need to exist (can be achieved by a for loop).

[LLFourn]

It looks like this was already sort of noted but we forgot to fix it: https://github.com/bitcoindevkit/bdk/pull/1041#discussion_r1347427162

[evanlinjin]

The caller will have to iterate over the changeset returned from batch_insert_relevant to see if last_seen and/or anchors are worth adding. Wouldn't that be cumbersome?

[LLFourn]

Good point. I hadn't considered that. I think we can at least remove batch_insert_unconfirmed from TxGraph and IndexedTxGraph.