Bluetooth: BAP: Shell: Fix issue with stopping broadcast sink

When the broadcast sink is stopped, the sink was also
set to NULL via

default_broadcast_sink.bap_sink = NULL;

However the lifetime of the broadcast sink does not
follow the state of the streams, and it still exists
afterwards. The broadcast sink can only be terminated
(deleted) via the term_broadcast_sink command.

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