foal should not keep any existing generators

[sebb <se...@gmail.com>]

I think it is misleading to keep any of the existing generators unless
they generate the same output for the same input as the existing code.

However this is going to be very hard to do without lots of special
case code in the archiver.

If there is a need to use the old generators with the updated
database, then it would be much simpler to write an amended importer
which invokes the original archiver, but writes to the new database
format.