[FIX] project: remove constrains for multilevel subtasks

The commit 79132f6 allowed multiple level for the subtasks, but a
remnant from the ancient time was left.

closes odoo/odoo#56104

Taskid: 2317929
X-original-commit: ace68ceceb92f8d7c16155fd72acd469424accd0
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/project/models/project.py

@@ -953,12 +953,6 @@ class Task(models.Model):
             else:
                 task.stage_id = False
 
-    @api.constrains('parent_id', 'child_ids')
-    def _check_subtask_level(self):
-        for task in self:
-            if task.parent_id and task.child_ids:
-                raise ValidationError(_('Task %s cannot have several subtask levels.', task.name))
-
     @api.returns('self', lambda value: value.id)
     def copy(self, default=None):
         if default is None: