Silence alert about large-statement-fetcher (#9882)

* Silence alert about large-statement-fetcher

That task is not continuous and behaving exactly as desired.

* Get rid of rule completely

- Pierre thinks it does not add any value
- Basti rightly complains that Polkadot specific exceptions make little
  sense in the Substrate repo.

.maintain/monitoring/alerting-rules/alerting-rules.yaml

@@ -133,16 +133,6 @@ groups:
   # Others
   ##############################################################################
 
-  - alert: ContinuousTaskEnded
-    expr: '(polkadot_tasks_spawned_total{task_name != "basic-authorship-proposer", task_name != "substrate-rpc-subscription"} == 1)
-        - on(instance, task_name) group_left() (polkadot_tasks_ended_total == 1)'
-    for: 5m
-    labels:
-      severity: warning
-    annotations:
-      message: 'Continuous task {{ $labels.task_name }} on node
-      {{ $labels.instance }} ended unexpectedly.'
-
   - alert: AuthorityDiscoveryDiscoveryFailureHigh
     expr: 'polkadot_authority_discovery_handle_value_found_event_failure /
     ignoring(name)