[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
Why 'employee_id' is related to the 'Expense Sheet' ???

Employees directly create expense lines. Then they can select their unsubmitted expense lines and submit them in batch to the HR, which will create the expense sheet.