Remove state and logic with no obvious effect

Since both branches in `write_` are identical, and there appears to be no other reference to this state, the ceremony around `entered_` seems to serve no purpose. Also, this property would be clobbered when entering and leaving **all** nodes, so it doesn't seem useful.

Also excuse me for skipping the [contributing](https://github.com/google/traceur-compiler/blob/master/CONTRIBUTING.md) hooplah for now - happy to go through it, but just interested in "what, if anything, I am missing" right now.