Bluetooth: Controller: df: Remove experimental for direction finding

Removes label experimental for direction finding feature in
Bluetooth Controller.

The feature stays disabled by default because it requires additional
dedicated hardware to be used. Besides that it significantly enlarges
amount of memory used by the Controller.

End user has to enable the feature explicitly by use of configuration
options.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>