mitaka-eol: Remove trusty from lib-forward-testing

All jobs have been removed already, adjust job-group to not define
unused jobs. With mitaka EOL we do not need to forward test on mitaka
anymore.

Change-Id: I2226909bfe8ba13e5f7be2f1328c66602c2b88c0

jenkins/jobs/tempest-jobs.yaml

@@ -3,9 +3,7 @@
     jobs:
       - '{pipeline}-tempest-dsvm-neutron-src-{name}-{node}{suffix}':
           pipeline: gate
-          node:
-            - ubuntu-trusty
-            - ubuntu-xenial
+          node: ubuntu-xenial
           suffix: ''
           branch-override: default