SCP-3501: Remove txs from chain index

[ak3n]

Pre-submit checklist:

• Branch
  • Tests are provided (if possible)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
  • Relevant tickets are mentioned in commit messages
  • Formatting, materialized Nix files, PNG optimization, etc. are updated
• PR
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

[koslambrou]

The implementation looks good. However, I would also add a property test to ensure this works as expected. Here's an idea.

In HandlerSpec, you can add a property test which:

• calls appendBlock
• for all possibles addresses (or at least one address), call utxoSetAtAddress
• Then call getUtxoutFromRef for each utxo
• Verify the result is not Nothing