refactor(wallet)!: Remove Wallet::cancel_tx function

[notmandatory]

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

/// 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:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

Bugfixes:

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

[ValuedMammal]

ACK 1854b8e

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.08%. Comparing base (c422104) to head (1854b8e).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
- Coverage   79.17%   79.08%   -0.10%     
==========================================
  Files          24       24              
  Lines        5311     5302       -9     
  Branches      242      241       -1     
==========================================
- Hits         4205     4193      -12     
- Misses       1029     1032       +3     
  Partials       77       77              

Flag	Coverage Δ
rust	79.08% <ø> (-0.10%)	⬇️

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