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 do not 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. Go to Setup > Product Configuration
  2. You'll land on the Message Catalog Editor tab
  3. Search the beginning part of the message "If you donate and have not already registered"
  4. You'll find the specific message that is appearing on your donation form
  5. If you want to edit the text, click Edit Custom. Enter your desired text and 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."