Bluetooth: Audio: Rename ctx type prohibited to none

The context type PROHIBITED has been removed from
the Bluetooth assigned numbers document. It is, however,
still used in some profiles to indicate either "No context
type supported" or "None available", and thus a rename
to "NONE" makes sense.

It is still treating similar to prohibited in some cases,
as it is not a valid context type in all cases.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>