[ANCHOR-445] Remove unique_address callback

[philipliu]

Description

• TODO: describe what this change does

Context

• TODO: describe why this change was made

Testing

• ./gradlew test
• TODO: replace with any additional test steps

Documentation

TODO: If this pull request adds a feature, describe changes to documentation that has been made

• Attach stellar-docs pull request, documenting new feature
• If it's an urgent feature request, please create a ticket and attach ticket number to this PR

Known limitations

TODO: describe any limitations or replace with N/A