Simplify code at a few places

Remove some duplicate checks,
and unnecessary errors.
The `_chooseAccessPolicy()` that we call here [already has](https://github.com/hypothesis/annotator/blob/c7d36572e27c6edb32c115d62e9195a67b75adb2/src/annotator.coffee#L140) the same test, so we can safely remove it from here.