[CI] Drop unused nodeset

It is required for infra cleanup.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-December/026408.html

Change-Id: I4ab7b637baee8fa2c40aae510dcefb3fa84eecd3

zuul.d/nodesets.yaml

 ---
-- nodeset:
-    name: kayobe-centos8
-    nodes:
-      - name: primary
-        label: centos-8
-
 - nodeset:
     name: kayobe-centos8s
     nodes: