SQL scripts should not manage transactions

This bug was fixed a while back, but the migration added for
multi-attach reintroduced it for its sqlite_downgrade file.

This patch removes it.

blueprint enable-mysql-connector

Change-Id: Iaec8713bf5cf36bcc97afbca8dd025cda9c2238d
Closes-Bug: 1368391

cinder/db/sqlalchemy/migrate_repo/versions/040_sqlite_downgrade.sql

@@ -1,5 +1,3 @@
-BEGIN TRANSACTION;
-
 CREATE TABLE volumes_v39 (
     created_at DATETIME,
     updated_at DATETIME,
@@ -84,4 +82,3 @@ INSERT INTO volumes_v39
 DROP TABLE volumes;
 ALTER TABLE volumes_v39 RENAME TO volumes;
 DROP TABLE volume_attachment;
-COMMIT;