Remove Java build job

Java implementation of monasca-api has been deprecated in Queens cycle
and is not maintained anymore. This change removes the CI job for
building Java artifact.

Change-Id: I5d31b46bc91c7ba270aeb28760996866081c7e07

.zuul.yaml

@@ -156,8 +156,6 @@
         - monasca-tempest-python3-java-cassandra
         - monascalog-python3-tempest
         - docker-build-monasca-service
-        - legacy-monasca-api-maven-build:
-            voting: false
     gate:
       queue: monasca
       jobs: