[ci] Remove ubi8 step (elastic#174257)

UBI8 builds were removed in
elastic#173873, but I missed the CI step
that verifies the docker context. This removes the step.

.buildkite/pipelines/artifacts.yml

@@ -51,16 +51,6 @@ steps:
         - exit_status: '*'
           limit: 1
 
-  - command: KIBANA_DOCKER_CONTEXT=ubi8 .buildkite/scripts/steps/artifacts/docker_context.sh
-    label: 'Docker Context Verification'
-    agents:
-      queue: n2-2
-    timeout_in_minutes: 30
-    retry:
-      automatic:
-        - exit_status: '*'
-          limit: 1
-
   - command: KIBANA_DOCKER_CONTEXT=ubi .buildkite/scripts/steps/artifacts/docker_context.sh
     label: 'Docker Context Verification'
     agents: