If ever you need to remove custom code/an embed link from an onboarding form, navigate to Configuration → Custom Data → Forms → Locate form → Update, scroll to custom form, erase the custom code/embed link and hit submit.

This can also be done from platform setup. Navigate to Configuration → Platform setup → Candidate/ Client Onboarding, scroll to custom content, hit the edit icon, remove link and hit refresh.

Custom Data View

Platform Setup View