Bluetooth: BAP: Fix missing remove of streams from slist

In the broadcast sink when the streams disconnected (without
the PA disconnecting), the streams were not properly
removed from the sink's list of streams. This allowed
the same stream to be added to the list twice, which
would cause a infinite loop as the stream would
point to itself as the next.

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