Remove ObjectID aliasing from the API.

[robertnishihara]

Note that this does not rip out all of the machinery for doing ObjectID aliasing. It just throws an exception if you try to do it.

[mehrdadn]

I thought aliasing was unavoidable...?

[robertnishihara]

Not unavoidable. Aliasing is desirable so that you can do things like this.

@ray.remote
def f():
  return 1

@ray.remote
def g():
  return f.remote()

However, things are simpler without it, so we're disabling it. If it seems critical for some applications, then we can add it back in later.