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/statement_of_applicability.go

@@ -85,7 +85,6 @@ FROM
 WHERE
     %s
     AND id = @statement_of_applicability_id
-    AND snapshot_id IS NULL
 LIMIT 1;
 `
 
@@ -132,7 +131,6 @@ FROM
 WHERE
     %s
     AND organization_id = @organization_id
-    AND snapshot_id IS NULL
     AND %s
 `
 	q = fmt.Sprintf(q, scope.SQLFragment(), cursor.SQLFragment())
@@ -169,7 +167,6 @@ FROM
 WHERE
     %s
     AND organization_id = @organization_id
-    AND snapshot_id IS NULL
 `
 	q = fmt.Sprintf(q, scope.SQLFragment())
 
@@ -252,7 +249,6 @@ SET
 WHERE
     %s
     AND id = @statement_of_applicability_id
-    AND snapshot_id IS NULL
 `
 
 	q = fmt.Sprintf(q, scope.SQLFragment())
@@ -293,7 +289,6 @@ DELETE FROM statements_of_applicability
 WHERE
     %s
     AND id = @statement_of_applicability_id
-    AND snapshot_id IS NULL
 `
 	q = fmt.Sprintf(q, scope.SQLFragment())