CI: Remove -rt suffix from kpet_tree_name values

Remove the deprecated -rt suffix from kpet_tree_name variable values in the .gitlab-ci.yml file.

Concerns: cki-project/pipeline-definition#211 (closed)

Concerns: redhat/centos-stream/tests/kernel/kpet-db#130 (closed)

Bugzilla: INTERNAL

Upstream Status: RHEL-only

Signed-off-by: Nikolai Kondrashov Nikolai.Kondrashov@redhat.com