Bluetooth: Remove _node field of bt_conn_cb if not used

After the #93703 PR was merged we get a possibility to remove _node field
from bt_conn_cb struct if the BT_CONN_DYNAMIC_CALLBACKS option is disabled.
The list conn_cbs can also be safely removed by more macro usage.

Signed-off-by: Radosław Koppel <r.koppel@k-el.com>