[Feature Flag] Remove "external_pipeline_validation_migration"

What does this MR do and why?

This reverts merge request !142077 (merged) as the FF was only used for migrating PVS from one URL (manual Cloud Run deployments) to another (Runway managed service). The external_pipeline_validation_service_url application setting has been updated in both staging & production so it should be safe to remove the FF now.

Issue: gitlab-com/gl-infra/platform/runway/team#175 (closed)

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