[CI] Drop Zuul host groups

They are not used.

Change-Id: Icea1a5f882e2e032c22d897efbd1f137536ae580
(cherry picked from commit e48d0a7fab)

zuul.d/nodesets.yaml

@@ -38,14 +38,6 @@
         label: ubuntu-focal
       - name: secondary2
         label: ubuntu-focal
-    groups:
-      - name: switch
-        nodes:
-          - primary
-      - name: peers
-        nodes:
-          - secondary1
-          - secondary2
 
 - nodeset:
     name: kolla-ansible-centos8-multi
@@ -56,14 +48,6 @@
         label: centos-8
       - name: secondary2
         label: centos-8
-    groups:
-      - name: switch
-        nodes:
-          - primary
-      - name: peers
-        nodes:
-          - secondary1
-          - secondary2
 
 - nodeset:
     name: kolla-ansible-focal-nested-virt