On most other forums I frequent, people just edit their own post and replace all the text with a single period, or perhaps with the phrase “(deleted by OP),” or something like that.
You can also ask a moderator to delete the post— as long as it isn’t the first post in a thread.
However, other members might still benefit from your post, even though you no longer have the same instrument, so you might reconsider whether you want to delete the post.