A subdomain is an additional part of your primary domain. Please, follow this tutorial to remove a subdomain in cPanel.
1. Log in to your cPanel account.
2. In the Domains section, click on Domains.
3. Locate the Subdomain you want to remove and click Manage next to it. 
4. Scroll down, click on Remove Domain, and confirm the removal.
You should see a message that your Subdomain has been removed.