You must have "Full Access" or "Limited Access - Registration Builder" permissions to edit registrations.
Navigate to the desired registration page and click Edit Mode.
On the top-right of the question, click the gray gear button.
Choose Edit.
Scroll down to the bottom of the question and click Show Advanced Options.
Under Advanced Options, check the box next to Hidden Question?
Click Save Question.
Hidden questions will only show for admins and not registrants.