Remove unsupported is-subset-of package

[ottenhoff]

No description provided.

[wheresrhys]

Thanks for doing this. I'm not too sure about accepting it as it is though. is-subset-of implements a lot more behaviour than your implementation, so there's a chance that this change could introduce some breaking changes. Also there are no tests, so even the subset of the previous behaviour that's been implemented might contain bugs. Previously I could get away with not writing detailed tests as I documented that I used a specific library, so users could look there for docs & issues, but if I'm to bring an implementation into this repo then it becomes my responsibility.

Do you know of any libraries that implement something like is-subset-of. If had a quick look around and can't find any.