refactor(core): Use /omit-if-no-ref/ for behaviors.

* Use lesser-known DT features to skip behaviors not referenced
  in the user keymap
* Update the behaviors to skip code if no nodes found.
* Remove some empty config/data structs where unused in
  behaviors.

app/src/behaviors/behavior_bt.c

@@ -17,6 +17,8 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 
 #include <zmk/ble.h>
 
+#if DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT)
+
 static int on_keymap_binding_pressed(struct zmk_behavior_binding *binding,
                                      struct zmk_behavior_binding_event event) {
     switch (binding->param1) {
@@ -49,3 +51,5 @@ static const struct behavior_driver_api behavior_bt_driver_api = {
 
 DEVICE_AND_API_INIT(behavior_bt, DT_INST_LABEL(0), behavior_bt_init, NULL, NULL, APPLICATION,
                     CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, &behavior_bt_driver_api);
+
+#endif /* DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT) */