doc: refer to not-up-to-date website: six-speed

[aghArdeshir]

https://nodejs.org/en/docs/es6/

In the link above there is a refrence to websit: six-speed. About performance of EcmaScript features. The website is so outdated.

[aghArdeshir]

[Trott]

I suspect the ES6 page should be removed entirely. It was a big and important thing when io.js forked and shortly after the merge back with Node.js. But it has been some time since it's been worth such a big call out, I think. Anyway, I'll open PR to remove the six-speed stuff. Thanks!