[Add,imp]:Remove expense 'sheet' concept

Task: https://www.odoo.com/web#id=9989&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.bd4JuNZUZclDd9oL
this means that if one line has been accepted, the sheet will be validated? this seems weird...