[CI] Remove UBI docker acceptance test (elastic#17828)

This commit removes the UBI acceptance test.
The default base image is UBI. Full flavour test has covered UBI flavour

Fixes: elastic#17830

Author: kaisecheng

.buildkite/scripts/exhaustive-tests/generate-steps.py

@@ -155,7 +155,7 @@ def acceptance_linux_steps() -> list[typing.Any]:
 
 def acceptance_docker_steps()-> list[typing.Any]:
     steps = []
-    for flavor in ["full", "oss", "ubi", "wolfi"]:
+    for flavor in ["full", "oss", "wolfi"]:
         steps.append({
             "label": f":docker: {flavor} flavor acceptance",
             "agents": gcp_agent(vm_name="ubuntu-2204", image_prefix="family/platform-ingest-logstash"),