Bluetooth: Audio: Remove caching ASE on release

Remove the support for caching ASEs on release.

This is mainly due to the 2-stage design in Zephyr: We
have the bt_audio_stream and the bt_audio_ep, where the
the latter represents an ASE in most cases.

However, once the endpoint goes into the codec configured
state, then a stream must be bound to it. This would be fine
(although a stream is technically not established at this point),
except that a stream is also, for unicast, specified for a
specific ACL. Once the ACL disconnects, the stream technically
becomes invalid, and if the stream is invalid, so is the endpoint.

Based on that, there's no reason (or even option) to keep the
endpoint in a codec configured state, as it would not
have a valid stream associated with it.

If we want to support an ASE going into the codec
configured state on ASE release (or ACL disconnect),
the relationsship between ASEs and streams needs to
be redefined.

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