CI: drop not used Xenial nodeset definitions

Change-Id: I96c477063338c48b033694dd85482a8723fd498b

zuul.d/nodesets.yaml

@@ -17,36 +17,12 @@
       - name: primary
         label: debian-buster-arm64
 
-- nodeset:
-    name: kolla-ansible-xenial
-    nodes:
-      - name: primary
-        label: ubuntu-xenial
-
 - nodeset:
     name: kolla-ansible-bionic
     nodes:
       - name: primary
         label: ubuntu-bionic
 
-- nodeset:
-    name: kolla-ansible-xenial-multi
-    nodes:
-      - name: primary
-        label: ubuntu-xenial
-      - name: secondary1
-        label: ubuntu-xenial
-      - name: secondary2
-        label: ubuntu-xenial
-    groups:
-      - name: switch
-        nodes:
-          - primary
-      - name: peers
-        nodes:
-          - secondary1
-          - secondary2
-
 - nodeset:
     name: kolla-ansible-bionic-multi
     nodes: