Due to domain name regulations, once a domain is registered, it remains active until its expiration date. That's why domain names can't be deleted from your account.
Once a domain reaches its expiration date, it enters a phase during which it may still be renewed. When this phase ends, the domain name will be automatically deleted.
Alternatively, if you wish to remove a domain from your hosting plan, follow these steps: How to Delete a Website From a Hosting Plan.