Remove remaining stylistic rules from ESLint config

[gaearon]

As discussed in #1948, there are still a few ESLint warnings about the style in our config. We should just remove them. If one wants to enforce style, they can just use Prettier.

[anilreddykatta]

@gaearon we want to get rid of all warn in our eslint? Do you have any specific set of rules? I can send a PR. 👍

[gaearon]

No, we keep the warn, but we remove those that are purely stylistic (i.e. warn about ugly spacing rather than real problems in the code). I think we only have one or two such rules, related to JSX.