[IMP] mrp: remove linked attachement on removing mrp document

Steps to Reproduce Bug:

- Add Attachment on Bom Line
- Delete Attachment

Bug:
- Attachment is still present on `ir.attachment`

With this commit, we are removing linked attachment on deleting `mrp.document`

closes odoo/odoo#74595

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>

addons/mrp/models/mrp_document.py

@@ -22,3 +22,7 @@ class MrpDocument(models.Model):
         ('1', 'Low'),
         ('2', 'High'),
         ('3', 'Very High')], string="Priority", help='Gives the sequence order when displaying a list of MRP documents.')
+
+    def unlink(self):
+        self.mapped('ir_attachment_id').unlink()
+        return super(MrpDocument, self).unlink()