In a discussion I found, a Microsoft MVP recommended the use of this program, "unassoc.exe", perfect for your needs.
unassoc
is a small portable executable, whose only UI elements include a list of all file types with associations on your computer, the ability to select one or more extensions, and the ability to delete the file type associated with that extension.
Also, if you're simply worried about the overhead of having multiple file extensions registered to a program you don't have installed anymore, the registry cleaner bundled with CCleaner can automate this process for you. It can scan for unused file extensions, and delete the relevant registry entries.
I do not want to change the filetype association to another program - just remove it.
So what do you want to happen when you double-click it? Do you want the Open with… dialog? What is the extension? How did you create it in the first place? There are several ways and places that an extension can be registered, so getting rid of it will depend on how/where it was made.