Remove upgrade script callbacks - not to be used

[ethanfrey]

• Targeted PR against correct branch (see CONTRIBUTING.md)

Remove all upgrade scripts callbacks from the upgrade module, as we no longer use them, and use the upgrade manager instead

[aaronc]

I agree that we should remove the default script behavior.

I still think it would be useful to allow an OnUpgrade callback, that by default isn't set. It's not too hard and there might be uses for that which we can't foresee yet. What do you think?

[ethanfrey]

"Cases we don't forsee" are dangerous.
They lead to over-engineered software.

If there is a clear need, do it.
If not, we add the feature when it is needed.

The feature didn't work reliably to swap out binaries, so we go with the upgrade manager. Let's stick with one way and make it work well. Don't give people too much to shoot themselves in the foot with