Updated docs for node.js: no need in NODE_ENV

raven-node documentation (https://github.com/mattrobenolt/raven-node) now says:

> We don't infer this from NODE_ENV automatically anymore. It's up to you to implement whatever logic you'd like.

Therefore sentry docs do not need to mention NODE_ENV configuration anymore.
Merge pull request #1101 from arty-name/patch-1

Updated docs for node.js: no need in NODE_ENV