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/data_protection_impact_assessment.go

@@ -138,8 +138,6 @@ WHERE
 type (
 	DataProtectionImpactAssessment struct {
 		ID                          gid.GID                                     `db:"id"`
-		SnapshotID                  *gid.GID                                    `db:"snapshot_id"`
-		SourceID                    *gid.GID                                    `db:"source_id"`
 		OrganizationID              gid.GID                                     `db:"organization_id"`
 		ProcessingActivityID        gid.GID                                     `db:"processing_activity_id"`
 		Description                 *string                                     `db:"description"`
@@ -192,7 +190,6 @@ FROM
 WHERE
 	%s
 	AND organization_id = @organization_id
-	AND snapshot_id IS NULL
 `
 
 	q = fmt.Sprintf(q, scope.SQLFragment())
@@ -221,8 +218,6 @@ func (dpias *DataProtectionImpactAssessments) LoadByOrganizationID(
 	q := `
 SELECT
 	id,
-	snapshot_id,
-	source_id,
 	organization_id,
 	processing_activity_id,
 	description,
@@ -237,7 +232,6 @@ FROM
 WHERE
 	%s
 	AND organization_id = @organization_id
-	AND snapshot_id IS NULL
 	AND %s
 `
 
@@ -271,8 +265,6 @@ func (dpias *DataProtectionImpactAssessments) LoadAllByOrganizationID(
 	q := `
 SELECT
 	id,
-	snapshot_id,
-	source_id,
 	organization_id,
 	processing_activity_id,
 	description,
@@ -287,7 +279,6 @@ FROM
 WHERE
 	%s
 	AND organization_id = @organization_id
-	AND snapshot_id IS NULL
 `
 
 	q = fmt.Sprintf(q, scope.SQLFragment())
@@ -319,8 +310,6 @@ func (dpia *DataProtectionImpactAssessment) LoadByID(
 	q := `
 SELECT
 	id,
-	snapshot_id,
-	source_id,
 	organization_id,
 	processing_activity_id,
 	description,
@@ -371,8 +360,6 @@ func (dpia *DataProtectionImpactAssessment) LoadByProcessingActivityID(
 	q := `
 SELECT
 	id,
-	snapshot_id,
-	source_id,
 	organization_id,
 	processing_activity_id,
 	description,