Remove support for saving MemFile undo steps as .blend files [Proposal]

[ZedDB]

@brecht

In some way I still think it would be good if auto save could work based on the undo stack, and even work asynchronously in a background thread.

That sounds awesome to me as then we would not lock up the interface while saving. It could also solve the “changes won’t be saved in sculpt mode without toggling edit modes”.