This field was used for extend feature to get handle position of metastrip children. Since D9972 extend feature works only on meta strip itself, not it's children. So `SEQ_transform_get_left_handle_frame()` second argument is always false and can be removed. Another instance of `seq->tmp usage` is hack to distinguish strips to be shuffled, which is not covered by this patch. Reviewed By: campbellbarton Differential Revision: https://developer.blender.org/D10321