[FIX] crm: deleting a crm.activity

crm.activity inherits mail.message.subtype with the field subtype_id
Before rendering "chatter.ChatComposer", the internal subtypes are fetched
on model mail.message.subtype(in function "fields_view_get").
But when deleting a crm.activity, the linked mail.message.subtype was not
deleted.

Used case:

When deleting a crm.activity, the activity still appeared in the chatter
if you log an internal note for an opportunity as an employee.

opw:670654

addons/crm/models/crm_activity.py

@@ -45,3 +45,11 @@ class CrmActivity(models.Model):
         if 'internal' not in values and 'default_internal' not in self._context:
             values['internal'] = True
         return super(CrmActivity, self).create(values)
+
+    @api.multi
+    def unlink(self):
+        activities = self.search([('subtype_id', '=', self.subtype_id.id)])
+        # to ensure that the subtype is only linked the current activity
+        if len(activities) == 1:
+            self.subtype_id.unlink()
+        return super(CrmActivity, self).unlink()