Remove non-standard raw option

[vweevers]

https://github.com/Level/level.js/blob/e819338e87aa305e57ec9c2fe58168ab5dca6da3/index.js#L47-L53

[vweevers]

@ralphtheninja the idea of the upgrading label is to remember breaking changes?

[ralphtheninja]

@vweevers Exactly!

[vweevers]

In UPGRADING.md we can mention the id encoding, which is the "idiomatic" way to do what raw does here.