[Merged by Bors] - chore: code cleanup

[fasmat]

Motivation

I removed some dead code from the codebase and cleaned up some other issues my linter complained about.

Description

• Prepared changelog for changes of the next release
• un-exported errors that don't need to be exported
• removed assignments to _ if all return values are ignored
• removed dead code from the common/util package

Test Plan

existing tests pass

TODO

• [x] Explain motivation or link existing issue(s)
• [x] Test changes and document test plan
• [x] Update documentation as needed
• [x] Update changelog as needed

[fasmat]

bors merge

[spacemesh-bors[bot]]

Pull request successfully merged into develop.

Build succeeded:

• ci-status
• systest-status