You can't delete the question, but doing so also won't solve your problem anyway.
When you post a question on stackexchange, you agree in the terms and conditions that you don't have the right to delete the question. (You may only remove your name from it.) The question may be deleted by moderators, administrators, or the anti-spam bot if it is considered not to have any value, but questions with value can't just be removed by the user. People have taken the time to answer your question, so you shouldn't be able to make all their hard work worthless in a fit of pique (there have been problems with users deleting all their questions when ragequitting, and then the answers don't make sense any more). A moderator may make an exception for you based on the circumstances.
See also: How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?, under "When can't I delete my own post?"
Turnitin regularly scrapes content from various websites, including stackexchange. Now that turnitin knows about this text, it is in its database and won't be removed from there just because you delete the question on the site.
However, everyone knows that turnitin sometimes finds duplicate passages that are not instances of plagiarism. For instance, generally all references are highlighted, because other people have cited the same paper before. Another case is, as here, publication of the same text by the same author in another location.
Therefore, the solution is simply for you to add a note when you hand in your essay saying that you wrote this question.