fix(infra): remove dead Cockpit remote_write from GitLab Prometheus

The remoteWrite block had placeholder values (COCKPIT_METRICS_PUSH_URL,
COCKPIT_PUSH_TOKEN) that were never replaced, causing Prometheus to
spam "Failed to send batch" warnings every minute. Cockpit was replaced
by self-hosted Grafana+Prometheus+Loki+Tempo.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infra/k8s/gitlab/values.yaml

@@ -95,13 +95,6 @@ prometheus:
       limits:
         cpu: 500m
         memory: 1Gi
-    # remote_write to Cockpit — pass secrets via --set at install time:
-    #   --set "prometheus.server.remoteWrite[0].url=<metrics_push_url>"
-    #   --set "prometheus.server.remoteWrite[0].headers.X-Token=<push_token>"
-    remoteWrite:
-      - url: "COCKPIT_METRICS_PUSH_URL"
-        headers:
-          X-Token: "COCKPIT_PUSH_TOKEN"
 gitlab-runner:
   install: false