The "Do not allow Instructure to alert you when you delete a question" alert is not a feature of Canvas LMS. This pop-up was likely from your web browser or a browser extension, not from Canvas itself. By checking the box, you told your browser to stop showing pop-ups from the Canvas website. This action is what is now preventing you from deleting quiz questions. To fix this, you need to change your browser's settings to allow pop-ups for Canvas.
How to Reverse the Process:
Since the setting is in your browser, you'll need to adjust your browser's settings to allow pop-ups from Canvas again. Here's how to do it in some common browsers:
Google Chrome
Go to Settings > Privacy and security > Site Settings > Pop-ups and redirects.
Look for canvas.instructure.com in the "Blocked" list and remove it.