remove isNative?

[ianstormtaylor]

I haven't looked into this deeply, but I remember having a conversation where it seemed like the performance benefits we were getting from isNative might not be as much as we thought, given that we do some other performance-enhancing things that Draft.js does not (and isNative was borrowed from Draft).

So we might be able to get rid of it, which would simplify a lot of other things.

If someone wanted to investigate this with a PR and check the performance after removing it, that would be amazing!