[FIX] snailmail: use the correct activity type

The activity generated by the snailmail sending is actually an exception
and should therefore use the corresponding activity type.

addons/snailmail/data/snailmail_data.xml

@@ -11,11 +11,5 @@
             <field name="numbercall">-1</field>
             <field name="doall" eval="False"/>
         </record>
-
-        <record id="mail_activity_data_snailmail" model="mail.activity.type">
-            <field name="name">Snailmail</field>
-            <field name="icon">fa-address-card</field>
-            <field name="sequence">15</field>
-        </record>
     </data>
 </odoo>

addons/snailmail/models/snailmail_letter.py

@@ -245,7 +245,7 @@ class SnailmailLetter(models.Model):
                 activity_data = {
                     'res_id': letter.res_id,
                     'res_model_id': self.env['ir.model']._get(letter.model).id,
-                    'activity_type_id': self.env.ref('snailmail.mail_activity_data_snailmail').id,
+                    'activity_type_id': self.env.ref('mail.mail_activity_data_warning').id,
                     'summary': _('Post letter: an error occured.'),
                     'note': note,
                     'user_id': letter.user_id.id,