So, when self-hosted websites use plugin’s like Jetpack they are added to that users accounts.
When site’s go down, removed for good those accounts are still stuck with a visual of what use to be and it cannot be removed with an installation of WordPress on that domain (quiet difficult if the site no longer exists).
You can’t delete them but Staff can. Provided you post the URLs of self hosted blogs you want deleted from your account into a forum thread like this one, Staff can do this for you. Please do that now as I tagged this thread for their assistance. Please subscribe to it so you are notified when they respond.