Follow the steps to remove Subdomain from your cPanel:
Step::1 Login to your cPanel
Step::2 Go to Domains section and then click on Subdomains
Step::3 Click on the SubDomains icon and find the subdomain that you wish to remove.
Step::4 Go to the Actions column and then click on Remove in order to remove the domain.
Step::5 Confirm the action by clicking on Delete Subdomain button and it’s complete.