Removed preferred document base category since this is meant to be set at the level of portal_types in ERP5Type (although the idea of central setting was nice) and because use of parallel fields is probably not the most appropriate to set multiple base categories. (use proxy fields or yet to be finish form field).

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@13469 20353a03-c40f-0410-a6d1-a30d3c3de9de