QmlDesigner: Remove double lock

There is already a lock by an other transaction so we don't need this
anymore.

Change-Id: I0f97c41e2a680344e05fec3d02c199108e599a3a
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/designercore/imagecache/imagecachestorage.h

@@ -198,13 +198,9 @@ private:
 
         void updateTableToVersion1(DatabaseType &database)
         {
-            Sqlite::ExclusiveTransaction transaction{database};
-
             database.execute("DELETE FROM images");
             database.execute("ALTER TABLE images ADD COLUMN midSizeImage");
             database.setVersion(1);
-
-            transaction.commit();
         }
     };