chore: remove Drone pipeline and Dockerfile

Gitea Actions replaced Drone, and the Dockerfile packaged a Java 11 image
of a jar that runs on the Pi, not in Docker. The backend image comes with
the backend itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -1,4 +0,0 @@
-FROM amazoncorretto:11.0.17
-COPY target/test*.jar app/home-pump.jar
-
-CMD ["java", "-jar", "app/home-pump.jar"]