Bluetooth: ISO: Fix BIG cleanup

Fix a regression for cleaning up BIGs added by
80c824f18ee80caa5bd162b9d2957dde1639add6 where the way
ISO connections were unref'ed was changed.

bt_iso_cleanup has been changed to only effect CIS
(as the unref from that has been removed), so instead
of calling bt_iso_cleanup we just call
bt_conn_unref directly.

This also removes some unref's from the BIG complete
and BIG Sync complete event handlers as the BIS bt_conns as
they don't increase the ref counter before anymore.

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