Clean up, mostly of silly useless TODO comments.

Mostly by removing useless comments, but also removed the CircuitBreaker.
The CircuitBreaker was removed because it was not used anywhere in our
code, and everything under org.neo4j.kernel is private API, so deleting it
is safe since clients can only depend on it at their peril.

I left the FinalTraversalBranch class stay, even though it too is really
suspect. The only place where it is used is obviously broken, and might not
even be used. I might do more research to look into this, to see if it too
can be removed, or if we actually wanted to use it for something.