Discontinue ondisk2 support

[spanezz]

Now that all known production datasets have been migrated from type=ondisk2 to other dataset types, we can dismiss ondisk2 support, and reduce the amount of code we need to maintain.

If some dataset is found still using ondisk2, it can always be converted by changing its configuration, removing the index, and running arki-check -f to cause segments to be reindexed.

A minimal awareness of ondisk2 needs to remain just to give helpful error messages, like: "ondisk2 type dataset are not supported anymore, please convert the dataset to iseg type"