[FIX] project: fix multi level subtask

- Create a project
- Create 3 tasks in the project (top, middle, bottom)
- Set top as the parent of middle
- Set middle as the parent of bottom

We don't want to allow multi level subtasks. The behavior is not the
same if you set middle as the parent of bottom first. The "parent task"
field will not be displayed.

This commit remove the tasks who have a parent from the m2o field.

OPW-2087921

closes odoo/odoo#39718

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>