Drop old unused tables for prometheus alerts

Description

Drop tables:

• issues_prometheus_alert_events
• issues_self_managed_prometheus_alert_events

See #442748 (comment 1830484684) and Respond: Create & link model classes for tables... (#442748 - closed) for details. The application code corresponding to these tables should also removed.

Implementation Plan

How to drop tables in GitLab?

• https://docs.gitlab.com/ee/development/migration_style_guide.html#dropping-a-database-table
• The tables have foreign keys, so use a post-deploy migration (probably one migration per table)
• https://docs.gitlab.com/ee/development/database/post_deployment_migrations.html
• Remove the table doc files
  • db/docs/issues_prometheus_alert_events.yml
  • db/docs/issues_self_managed_prometheus_alert_events.yml
• Similar merge request for reference !153443 (merged)