[FIX] calendar: event of done activity must not be deleted

To reproduce
============
- Create a meeting activity from any document (for example CRM opportunity) with a calendar.
- It will create a meeting in the calendar.
- Now, mark as done the activity created and it will delete the meeting from the calendar too.

revert of https://github.com/odoo/odoo/pull/144526

opw-3626773

closes odoo/odoo#146421

X-original-commit: 0b183299a8ef7361d28d9d2b49cc56b74bfb2b46
Signed-off-by: Abdelouahab Laaroussi (abla) <abla@odoo.com>

addons/calendar/models/mail_activity.py

@@ -39,10 +39,7 @@ class MailActivity(models.Model):
         return super(MailActivity, self)._action_done(feedback=feedback, attachment_ids=attachment_ids)
 
     def unlink_w_meeting(self):
-        return self.unlink()
-
-    def unlink(self):
         events = self.mapped('calendar_event_id')
-        res = super().unlink()
+        res = self.unlink()
         events.unlink()
         return res

addons/calendar/tests/test_mail_activity_mixin.py

@@ -77,3 +77,6 @@ class TestMailActivityMixin(MailCommon):
             self.assertEqual(test_record.activity_calendar_event_id.name, ev1.name, "This should be the calendar event of the next activity")
 
             act1._action_done(feedback="Mark activity as done with text")
+
+            self.assertFalse(act1.exists(), "activity marked as done should be deleted")
+            self.assertTrue(ev1.exists(), "event of done activity must not be deleted")