Stop testing on ubuntu-xenial

We've switch to ubuntu-bionic since it is stable, this also allows us to
save some CI resources.

Change-Id: I7f9d5201b4a19c976b8e3243dfd4e7a1a72d6456
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

.zuul.yaml

@@ -14,25 +14,17 @@
     parent: ansible-role-grafyaml
     nodeset: ubuntu-bionic
 
-- job:
-    name: ansible-role-grafyaml-ubuntu-xenial
-    parent: ansible-role-grafyaml
-    nodeset: ubuntu-xenial
-
 - project:
     templates:
       - windmill-jobs-fedora-latest
       - windmill-jobs-bionic
-      - windmill-jobs-xenial
     check:
       jobs:
         - ansible-role-grafyaml-fedora-latest
         - ansible-role-grafyaml-ubuntu-bionic
-        - ansible-role-grafyaml-ubuntu-xenial
         - tox-linters
     gate:
       jobs:
         - ansible-role-grafyaml-fedora-latest
         - ansible-role-grafyaml-ubuntu-bionic
-        - ansible-role-grafyaml-ubuntu-xenial
         - tox-linters