Automatic merge from submit-queue

examples/guestbook: remove not useful/inaccurate comments

The comments removed in this patch are either inaccurate or
does not serve any purpose as they're already applies as uncommented
in the tutorial and explained in the walkthrough text.

```release-note
NONE
```
Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>