Bluetooth: Replace le_auto_conn debugfs with device_list entry

Since the auto-connection handling has gained offical management
command support, remove the le_auto_conn debugfs entry.

For debugging purposes replace it a simple device_list debugfs
entry that allows listing of the current internal auto-connection
list used for passive scanning.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>