Bluetooth: iso: remove unused struct 'iso_list_data'

'iso_list_data' has been unused since the original
commit ccf74f23 ("Bluetooth: Add BTPROTO_ISO socket type").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>