Remove automated creation of importer / input format on upgrade

When upgrading from 6.x-1.x to 6.x-2.x, a new input format and feeds importer are currently created for each existing mailbox. This is really inefficient and not perfect, since commands can't be converted and importers all get attached to one content type. A better alternative would be to create an UPGRADE.txt with instructions on how to create an input format and customize the default importer to use the format and map commands, etc...