SSH: Remove questionable warning in connection manager.

If there are still open channels in a connection being released,
that is not necessarily a mistake: The channels could already be
in the closing state, and it would be unreasonable for us to expect
API clients to always wait for confirmation before releasing the
connection, as this can complicate code quite a bit, turning a
synchronous operation into an asynchronous one.

Change-Id: If8c604c9ee1294728e7947c8d5c8130d6e704b49
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>