There's no way to delete anything you post, but ladymcbeth gave the best suggestion, just disavow your question.
The Admins no longer remove questions by request anymore, whether you message them or report it, unless it breaks the posting guidelines or you have a very good reason, it will not be taken down.
To disavow one of your questions, view your questions page, find the question you don't want anymore, and hover over the broken chain link icon on the right side of the page. A bubble should come up asking if you want to disavow it.
After you disavow a question, it will disappear from your questions page, be permanently set to anonymous, you won't get any notifications if anyone comments on it, and you won't be able to get the question back later.
You cannot delete questions and the admins cannot delete them unless they violate the guidelines. You can only disavow them which means the question goes in anonymous mode, it gets removed from your profile and you won't get further notifications for it.