1854b8e cleanup(wallet)!: remove cancel_tx function (Steve Myers)

Pull request description:

  ### Description

  Per discussion in team chat today I'm removing the `Wallet::cancel_tx` function because it's name is misleading and the `Wallet::mark_unused` function can be used to accomplish the same function.

  closes #326 and closes #212 and closes #41

  ### Notes to the reviewers

  This is an example of how to use `mark_unused` instead of `cancel_tx`

  ```rust
  /// This marks as unused the change addresses used when creating a tx so they can be used in future transactions.
  for txout in &tx.output {
      if let Some((keychain, index)) = txout_index.index_of_spk(txout.script_pubkey.clone()) {
          // NOTE: unmark_used will **not** make something unused if it has actually been used
          // by a tx in the tracker. It only removes the superficial marking.
          wallet.unmark_used(*keychain, *index);
      }
  }
  ```

  ### Changelog notice

  - BREAKING: Remove Wallet::cancel_tx function. Use Wallet::unmark_used instead

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `just p` before pushing

  #### Bugfixes:

  * [x] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  ValuedMammal:
    ACK 1854b8e

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