Hi Michael,
Your feedback makes perfect sense; I can understand why you wouldn't want that particular link to show up when third parties view the report email.
Fortunately, that link only appears in the default email text. If you personalize your Custom Report's email message, your custom text will replace everything in the default message except for the essential "download report" link.
Check out these instructions for more information on setting up Custom Reports. At the bottom of that help page is a screenshot of the "Schedule & Share" step, showing where you have the ability to customize the email body message. Enter any text you like in order to replace the default messaging.
Hope this helps!