Remove deprecated snapshot system

The register/document model has fully replaced the snapshot system.
Delete every snapshot-scoped row and strip the application code that
referenced them: SnapshotID/SourceID struct fields, snapshot_id IS NULL
filters, snapshot columns from SELECT/INSERT statements and named args,
and the eight migrate-*-snapshots-to-documents one-shot tools.

The remaining snapshot_id / source_id columns, the snapshots and
controls_snapshots tables, the snapshots_type enum, and the
snapshot-scoped indexes are now unused; they are dropped in a follow-up
schema migration so this change can roll back cleanly without leaving
orphaned data.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/coredata/processing_activity_third_party.go

@@ -29,7 +29,6 @@ type (
 		ProcessingActivityID gid.GID      `db:"processing_activity_id"`
 		ThirdPartyID         gid.GID      `db:"third_party_id"`
 		TenantID             gid.TenantID `db:"tenant_id"`
-		SnapshotID           *gid.GID     `db:"snapshot_id"`
 		CreatedAt            time.Time    `db:"created_at"`
 	}