refactor: remove explicit operator!= using C++20 rewrite candidates

[mapleFU]

C++20 supports rewrite candidates, see https://en.cppreference.com/w/cpp/language/overload_resolution.html#Call_to_an_overloaded_operator

For x != y, all member, non-member, and built-in operator==s found are added to the set, unless there is a matching operator!=.

[Fokko]

Less is more, thanks @mapleFU for fixing this, and thanks @lidavidm and @wgtmac for the review 🚀