[danfossairunit] Provide more elegant deprecation of channel main#manual_fan_speed

[jlaur]

#11147 renamed a channel for consistency and left the old channel for backwards compatibility - "hidden" as an advanced channel.

This was a bit awkward and I've since then learned how this could be achieved in a bit more elegant way. So with this issue I plan to provide a pull request for removing the deprecated channel if no items are linked. If any items are linked, a warning should be logged.