Undeprecate deregister() and chanelUnregistered()

Motivation:

As discussed in #2250, it will become much less complicated to implement
deregistration and reregistration of a channel once #2250 is resolved.
Therefore, there's no need to deprecate deregister() and
channelUnregistered().

Modification:

- Undeprecate deregister() and channelUnregistered()
- Remove SuppressWarnings annotations where applicable

Result:

We (including @jakobbuchgraber) are now ready to play with #2250 at
master