[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
One more thing, employee_id in hr_expense.sheet is mandatory, here you are not passing employee_id, are you sure you'll always have default employee, check method _employee_get, it may return False if current uid not having related employee