Bluetooth: ISO: Remove bt_conn_unref for ISO deferred work

Removed the bt_conn_unref from the deferred_work function.
For ISO, the conn unref for the peripheral will happen in
the bt_iso_disconnected function. For the central, the
unref shall only happen when the CIG is terminated.

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