chore: remove unused impl of private trait ActiveSet

[RobWalt]

• [x] I agree to follow the project's code of conduct.

• [x] ~I added an entry to CHANGES.md if knowledge of this change could be valuable to users.~

The motivation for removing this is that it is a bit misleading and confusing when starting to look deeper into the code. I spent quiet some time trying to wrap my head around why we need this but it was simply unused.

This seem to be leftover parts of an earlier implementation. The trait is private, so there is no chance that this will break user code. I also made sure to check that the trait impl for BTreeSet isn't used anymore. It has been completely replaced by VecSet everywhere.

[rmanoka]

Yes, it's a left-over from a previous implementation but I wasn't sure if we'll switch back to BTreeSet so left it in there. We can remove this if it helps with some linting.