Blender Git Loki

UI Code Quality: Cleanup ui_but_update_from_old_block

This commit contains some improvements to this function to make this
function more purposeful and readable.
- Split updating information of the old button to a new function.
- Remove some 7 year old code disabled with `#if 0`.
- Add comments explaining some of the less obvious aspects.

Differential Revision: https://developer.blender.org/D9117