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/dts/behaviors/bluetooth.dtsi

@@ -6,7 +6,7 @@
 
 / {
 	behaviors {
-		bt: behavior_bluetooth {
+		/omit-if-no-ref/ bt: behavior_bluetooth {
 			compatible = "zmk,behavior-bluetooth";
 			label = "BLUETOOTH";
 			#binding-cells = <2>;