Add GovRemovedVotes event

[kderme]

Description

An attempt for #4633.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

It is a good first attempt, but there are a couple of things I don't quite like about it:

• It makes the addition of votes quite a bit more complicated, which is a dangerous thing and could lead to bugs in the future. At the very least it makes the logic for addition of votes so much harder to read
• It could affect performance of addition of votes, due to a more complex accumulator and some extra indirections

There is a better way to tackle this and we already have a very similar logic for figuring out known voters, which could be used for this purpose as well. I was not sure how to exactly suggest it, so I've just implemented it in this commit, so feel free to steal into your PR: 87e9c94

[lehins]

Looks great! 😃

[teodanciu]

Looks good to me

[lehins]

Unfortunately, this breaking change didn't make it into the latest 1.17.0.0 release, so we need to bump it to 1.18.0.0

[lehins]

Perfect. Thank you!