[IMP] project: Do not set date_assign in default get

Currently when calling default_get (example: Create a new task), if a user is set on the new task, the assign date is automatically set at now()

It seems wrong for 2 reasons:
- It's alreay done either in write or create
- If the user creates a new task, but removes himself from the `Assigned to` field, the assignation date is still set, which is wrong.

addons/project/models/project.py

@@ -320,14 +320,6 @@ class Task(models.Model):
     _mail_post_access = 'read'
     _order = "priority desc, sequence, date_start, name, id"
 
-    @api.model
-    def default_get(self, field_list):
-        """ Set 'date_assign' if user_id is set. """
-        result = super(Task, self).default_get(field_list)
-        if 'user_id' in result:
-            result['date_assign'] = fields.Datetime.now()
-        return result
-
     def _get_default_partner(self):
         if 'default_project_id' in self.env.context:
             default_project_id = self.env['project.project'].browse(self.env.context['default_project_id'])