fix: clean up app.yaml by removing unused variables section

Author: dido18

internal/orchestrator/bricks/testdata/dummy-app/app.yaml

@@ -1,8 +1,6 @@
 name: Copy of Blinking LED from Arduino Cloud
 description: Control the LED from the Arduino IoT Cloud using RPC calls
+icon: ☁️
 ports: []
 bricks:
-- arduino:arduino_cloud:
-    variables:
-      ARDUINO_SECRET: a-secret-a
-icon: ☁️
+- arduino:arduino_cloud: