[FIX] project: default context key

- Activate Sub-tasks in projects
- Create a sub-task

An `AccessError` is raised in `check_access_rule` of `mail.message`.

The context is 'polluted' with a `default_parent_id` key, which is not
necessary anymore at this point.

opw-1924899

closes odoo/odoo#31022

addons/project/models/project.py

@@ -680,6 +680,7 @@ class Task(models.Model):
         # force some parent values, if needed
         if 'parent_id' in vals and vals['parent_id']:
             vals.update(self._subtask_values_from_parent(vals['parent_id']))
+            context.pop('default_parent_id', None)
         # for default stage
         if vals.get('project_id') and not context.get('default_project_id'):
             context['default_project_id'] = vals.get('project_id')