deployment: drop observer_applicable label in alert_builder (#14218)

PR #13938 ("apollo_dashboard: observer applicability affects expression")
moved observer applicability into the alert PromQL expression and removed
the field from crates/apollo_dashboard/resources/dev_grafana_alerts.json
(83 occurrences -> 0), but alert_builder.py was not updated and still
references dev_alert["observer_applicable"], crashing every deployment
that generates monitoring resources with KeyError: 'observer_applicable'.

Drop the dead label so monitoring generation succeeds at tags
APOLLO-0.14.3-RC.0 / RC.1 and on main.

Author: amit-chaet-starkware

deployments/monitoring/src/builders/alert_builder.py

@@ -318,7 +318,6 @@ def alert_builder(args: argparse.Namespace):
                 datasource_uid=args.datasource_uid,
                 labels={
                     "og_priority": dev_alert["severity"],
-                    "observer_applicable": dev_alert["observer_applicable"],
                 },
             )
         )