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

openstack-infra/project-config,openstack-infra/project-config