Remove old migration system.

[samwho]

Description

This PR removes the old global migration system.

As far as I was able to tell, this is not used anymore. We have a newer migration system that works per-app, and confusingly the endpoint to check the status of these new migrations was intermingled with endpoints that operated on the old migration system.

Launchcontrol

• Removes an old migration system. The following internal endpoints will no longer be present:
  • POST /api/migrations/run
  • GET /api/migrations/status
  • POST /api/system/migrations/run
  • GET /api/system/migrations/definitions

[qa-wolf]

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