Remove disabled Alerts section from 'Settings > Integrations > Prometheus'

Motivation

#219142 (closed) moved the configuration for alerts endpoints from Settings > Integrations > Prometheus to Settings > Monitor > Alerts. The fields left behind were disabled.

Proposal

1. Remove the deprecation banner on Settings > Integrations > Prometheus. Alternatively, at least update the text so it does not refer to Operations settings (rather than Monitor settings).
2. Remove the alerts section from Settings > Integrations > Prometheus.
3. Double-check for any links which send users to Settings > Integrations > Prometheus to configure the alerts endpoint and route to Settings > Monitor > Alerts instead
4. Rework documentation, for example:
   • https://docs.gitlab.com/ee/operations/metrics/alerts.html#external-prometheus-instances
   • https://docs.gitlab.com/ee/operations/incident_management/integrations.html#external-prometheus-integration

Banner to remove	Section to remove