Fix mvn shell script

We use bazel on the master branch now.

Buck is now unsupported.

See failure at https://gerrit-ci.gerritforge.com/job/plugin-manifest-subscription-mvn-master/

Change-Id: Iadf315a3a3ece3549e357fdc6777d9549b8f243e
diff --git a/jenkins/gerrit-mvn-build-plugin.sh b/jenkins/gerrit-mvn-build-plugin.sh
index 846ec57..aca5de3 100644
--- a/jenkins/gerrit-mvn-build-plugin.sh
+++ b/jenkins/gerrit-mvn-build-plugin.sh
@@ -4,12 +4,11 @@
 if [ "{branch}" == "master" ]
 then
   git read-tree -u --prefix=gerrit gerrit/{branch}
-  SOURCE_LEVEL=$(grep "source_level" gerrit/.buckconfig || echo "source_level=7")
-  . set-java.sh $(echo $SOURCE_LEVEL | cut -d '=' -f 2 | tr -d '[[:space:]]')
+  . set-java.sh 8
 
   pushd gerrit
-  buck build api
-  ./tools/maven/api.sh install buck
+  bazel build api
+  ./tools/maven/api.sh install
   popd
 fi