[fleet] New API migration

- Task: https://www.odoo.com/web#id=10075&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.xFp6558VZvondGaA
no need of this method.
There's a method specifically for str to datetime: https://github.com/odoo/odoo/blob/8.0/openerp/fields.py#L1121