Remove final pieces of AlertsService

As part of finally removing the deprecated AlertsService, we need to complete a few final steps:

• Remove app/models/project_services/alerts_service.rb & corresponding spec
• Remove alerts from Project#disabled_services
• Drop alerts_service_data table & foreign keys https://docs.gitlab.com/ee/development/migration_style_guide.html#dropping-a-database-table
• Possibility: Remove any new AlertsService records from services table (there shouldn't be any new ones, but it doesn't hurt to play it safe; we should definitely double check staging & prod)

The table removal should happen in the milestone after #255501 (closed) / !50913 (merged) is completed.