This commit removes the F-Curve handling from the legacy Curve edit mode
undo code.
The legacy Curve edit mode uses the per-mode undo system. This tried to
deal with changes in the animation data as well, but did so in a very
limited way. Only F-Curves were copied & restored, which means the
Action Groups were effectively deleted as soon as you undo something.
Furthermore, the code wasn't updated for slotted Actions yet, so
effectively it was a no-op, because it only saw the always-empty legacy
`Action::curves` field.
Fixing this properly would mean discarding this undo code, and moving to
the global undo system, as the animation editors allow changing the
animation of anything in the scene, not just the Curve data-block that
is being edited. Since the legacy curve ID type is, well, legacy, I
don't think it's worth going this route.
This is a non-functional change, as `action->curves` is always empty,
and so nothing was backed up or restored anyway.
Pull Request: https://projects.blender.org/blender/blender/pulls/135585
b71fed904b