I think your question is answered on Meta StackExchange. However, a key statement there is:
Moderators really shouldn't be removing decent content simply because the OP doesn't want it anymore.
I would say that simply worrying about your rep is likely not a compelling reason for your post to be deleted. As described here, by posting you have granted a license to display your post.
As described on both of those questions, you should contact StackExchange employees and/or site moderators if you want to have your post removed. However, my advice is to not do this if you're just worried your question will lower your rep. If you think your question is bad, edit it and make it better. Over time, if you continue to post useful questions and answers, negative rep from that one question will not be major problem. (As described on those questions, the main reason for deleting stuff that can't be deleted under normal site rules is if you post some sensitive information that needs to be removed to protect your privacy.)