Unfortunately, there is no way to remove this from the alert emails that SharePoint sends. You can edit the XML templates of the alert emails SharePoint sends, but SharePoint will always attach the alert-related email headers:
x-alerttitle: Daily Summary Notification
When Outlook sees those headers, it treats the mail item as a SharePoint alert.
The alternative would be to send alerts from a workflow or other method of generating emails. There isn't anyway to get rid of those for actual SharePoint alerts.