This verbiage is appearing because the option to opt in to email communication is hidden from the user and by donating, they are consenting to receive email. You can change the wording of this message, but cannot remove the text altogether. The message must clearly state that they are giving your organization permission to send email communications.

If you are in a group with the Advanced Administrator permissions, then you should have access to the Message Catalog Editor where you will find this text. Follow the steps below to find it.
  1. Navigate to Setup > Product Configuration
  2. You'll land on the Message Catalog Editor tab
  3. Canadian clients, please choose English (CA) from the Language drop down menu on the left hand side of the screen and click Refresh List >>
  4. In the search field, enter the beginning part of the message "If you donate and have not already registered"
  5. Click Search
  6. You'll find the specific message that is appearing on your donation form
  7. Under the Actions column, click Edit Custom
  8. Enter your desired text
  9. Click Submit
Note: Removing this message is in violation of our Email Acceptable Use Policy, specifically this caveat: "At the point of email address collection, Client will describe the nature of the email end users will receive and the organizational identities (e.g., domain names or brands) which Client will use to send email."