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
