Bluetooth: BAP: Broadcast Sink should not terminate the PA Sync

When calling bt_bap_broadcast_sink_delete, the broadcast sink
should not attempt to terminate the PA Sync. The PA sync can live
on without the broadcast sink, just as the broadcast sink can live
on without the PA sync (which is why the PA sync check was completely
removed).

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