[CI] Drop CentOS Linux 8 job and nodeset

CentOS Stream 8 (aka CentOS 8 Stream) is the currently
supported runtime platform. [0]

DevStack works with CentOS Stream only now. [1]

The only usage of the nodeset being dropped is handled by the
Depends-On.

[0] https://governance.openstack.org/tc/reference/runtimes/xena.html
[1] https://review.opendev.org/c/openstack/devstack/+/759122

Depends-On: https://review.opendev.org/c/openstack/cinder-tempest-plugin/+/795159
Change-Id: Ic0f696b46dce3dba529b53a8f9de8cda6b913c7b
1 file changed