You can remove your custom domain from within the builder. Please follow the steps below:
1. First unpublish your landing page by going to Setup > General > Status (click once so it chooses draft)
2. After that, go to Domain > Custom Domain, then delete what's on the text box:
3. Once it's deleted, go back to General > Status and click once again so it shows published, then a pop up box will show up. Just choose continue.