The purpose of Stack Exchange is to build a repository of answers that will help other people. So the real question is, will your “dumb” question and its answer(s) help others? If you think your question is dumb because you made a mistake, is this a mistake that others are likely to make? If you think your question is dumb because you forgot an “obvious” fact, is this something that others confronted with the same problem might also forget?
“Typo” questions — when the answer turns out to be “you made a trivial error in your code” or “you forgot to carry the 3” — are only useful to the original askers and should be deleted. Some Stack Exchange sites have a custom close reason for such cases, for example on Stack Overflow:
This question was caused by a problem that can no longer be reproduced or a simple typographical error. While similar questions may be on-topic here, this one was resolved in a manner unlikely to help future readers. This can often be avoided by identifying and closely inspecting the shortest program necessary to reproduce the problem before posting.
This case doesn't come up often enough here on Cryptography to dedicate a site-specific close reason to it, but you can use write-in close reasons.
On the other hand, if your question was based on a flawed line of reasoning, or if you forgot that a general result would provide a simple answer to your question, this often leads to answers that can help others studying the same problem. Often they highlight a subtlety in the concepts behind the question. These threads should stay.
I think your example falls into the second category.
In principle, you may delete your own question if it has no useful answer. Since the system cannot automatically determine the existence of a useful answer, an approximation is applied: you can delete your own question if it has no answer, or if it has a single answer with a score of at most 0. Unregistered users cannot delete their own question at all. Once your question has answers, it isn't yours to unilaterally delete any more; you need to go through the close-delete process.