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/third_party_risk_assessment.go

@@ -189,7 +189,6 @@ FROM
 WHERE
     %s
     AND third_party_id = @third_party_id
-    AND snapshot_id IS NULL
 ORDER BY
     created_at DESC
 LIMIT 1;
@@ -240,7 +239,6 @@ FROM
 WHERE
     %s
     AND third_party_id = @third_party_id
-    AND snapshot_id IS NULL
 	AND %s
 `
 
@@ -292,7 +290,6 @@ FROM
 WHERE
     %s
     AND third_party_id = ANY(@third_party_ids)
-    AND snapshot_id IS NULL
 ORDER BY
     third_party_id, created_at DESC
 `