You can’t delete a zone using Cloudflare Registrar since the domain would then be useless as you can’t change the nameservers which always use Cloudflare.
Do you want to remove the domain registration, or just move away from Cloudflare?
If moving away from Cloudflare, then transfer the domain out to another registrar. The change of nameservers will cause the zone to be automatically purged from Cloudflare after 14 days (ensure to downgrade to a free plan first).
If you want to delete the domain registration, go to “Domain Registration”, then “Manage Domains”, select the domain and then “Configuration” and if the domain registration can be deleted you should see…
Alternatively, make sure you are on a free plan, delete the DNS records and ensure the domain registration is not set to auto-renew and let it expire when it will also then purge from Cloudflare.