mesh: Fixes wrong app_key_idx parameters

`app_key_del` first param should be key_app_idx,
not key_net_idx.

The effect is that app_key_del is broken.

It's a regression in eca0141
Bluetooth: Mesh: Isolate cryptographic material

This is port of 779fd135f3395d5423bd508a5f31899d6d189d16
diff --git a/nimble/host/mesh/src/cfg_srv.c b/nimble/host/mesh/src/cfg_srv.c
index f82abd6..8a085bd 100644
--- a/nimble/host/mesh/src/cfg_srv.c
+++ b/nimble/host/mesh/src/cfg_srv.c
@@ -392,7 +392,7 @@
 
 	BT_DBG("AppIdx 0x%04x NetIdx 0x%04x", key_app_idx, key_net_idx);
 
-	status = bt_mesh_app_key_del(key_net_idx, key_net_idx);
+	status = bt_mesh_app_key_del(key_app_idx, key_net_idx);
 
 	send_app_key_status(model, ctx, status, key_app_idx, key_net_idx);
 }