[FIX] mail: discuss unstar all

Before this commit, when clicking on "Unstar All" from Discuss
'Starred' mailbox, it crashes with following error:

```
TypeError: unstar_all() takes 1 positional argument but 2 were given
```

This commit fixes this issue

closes #37994

closes odoo/odoo#38226

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>