Remove/hide "temporarily forwarding to stable" message for stable Ubuntu channels

[Dan Watkins]

When doing a manual `snap refresh` on an official Ubuntu Cloud system which has a snap pre-seeded in the image, I got the following output:

```
2018-12-11T21:36:29Z INFO Waiting for restart...
core 16-2.36.2 from Canonical✓ refreshed
google-cloud-sdk 228.0.0 from Cloud SDK (google-cloud-sdk✓) refreshed
Channel stable/ubuntu-18.04 for google-cloud-sdk is closed; temporarily forwarding to stable.
```

Per Ubuntu policy[0], pre-seeded snaps have to be pointed at a stable/ubuntu-$version branch. The language of the last line of output suggests that this is an unusual state of affairs, in which we find ourselves temporarily. This is not the case, and I think it will serve to give users the impression that snaps are not fully integrated with Ubuntu (when this message is in fact being output because this isn't the case!).

Is this wording something that we could look at improving (or, as it is simply reflecting the standard state of affairs, removing)?

[0] https://wiki.ubuntu.com/UbuntuSeededSnaps