Revert "[FIX] mail: remove attachment as main at unlink"

This reverts commit abc45b1
Since by default the ondelete attribute of a many2one is `set null`,
this was completely unnecessary to begin with.

Bug caused by this commit:
Unlink a record that has some attachments.
The unlink first removes the record, then its related attachments.
It calls remove_as_main_attachment, which reads the attachment res_model and
res_id. This triggers a check that the related record can be read.
However the related record has already been removed, an exception is raised.
It is thus impossible to unlink a record.

Closes #32563

closes odoo/odoo#32572

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>

addons/mail/models/ir_attachment.py

@@ -15,19 +15,6 @@ class IrAttachment(models.Model):
         for record in self:
             record.register_as_main_attachment(force=False)
 
-    @api.multi
-    def unlink(self):
-        self.remove_as_main_attachment()
-        super(IrAttachment, self).unlink()
-
-    @api.multi
-    def remove_as_main_attachment(self):
-        for attachment in self:
-            related_record = self.env[attachment.res_model].browse(attachment.res_id)
-            if related_record and hasattr(related_record, 'message_main_attachment_id'):
-                if related_record.message_main_attachment_id == attachment:
-                    related_record.message_main_attachment_id = False
-
     def register_as_main_attachment(self, force=True):
         """ Registers this attachment as the main one of the model it is
         attached to.