Remove comment attachment

[kaicataldo]

ESLint no longer uses attached comments in the AST and so babel-eslint doing comment attachment is now work that is just thrown away. I'd like to propose removing the behavior entirely (but still keeping the top-level comments array that ESLint does require to calculate comment location internally.

I opened a similar issue for the default parser, Espree.

Note that this is a breaking change, but it seems unlikely that any other project will really be affected by it since the purpose of this project is to provide compatibility with ESLint.