remove armv8a docker build. it doesn't work. looks like it never worked and no docker image was ever pushed to the docker registry. failing commands were ignored silently by travis.

.travis.yml

@@ -40,9 +40,3 @@ jobs:
               - docker login  -u $DOCKER_LOGIN -p $DOCKER_PASSWORD
               - docker build --pull -t plantuml/plantuml-server:tomcat -f Dockerfile.tomcat .
               - docker push plantuml/plantuml-server:tomcat
-        - stage: docker-push
-          name: armv8a
-          script:
-              - docker login  -u $DOCKER_LOGIN -p $DOCKER_PASSWORD
-              - docker build --pull -t plantuml/plantuml-server:armv8a -f Dockerfile.armv8a .
-              - docker push plantuml/plantuml-server:armv8a