If you need to delete a post, the best approach is to visit your message on the community, and select Report Inappropriate Content from the menu for that message. In the comments for the action, specify your request and ideally your reasons.
We don't allow members to delete their own messages; once the message is in the community, others may respond. Deleting a message (and its replies) will affect the contributions of others as well. Of course there are valid reasons for removing content, so we leave it as a moderator action that we can take quickly upon request.