A subdomain is an additional part of your primary domain. Please, follow this tutorial to remove a subdomain in Plesk.
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1. Log in to your Plesk account.
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2. Click on Websites & Domains from the Sidebar Menu.
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3. Under the list of Domain names, click on the More icon 
in front of the subdomain and select Remove subdomain.
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4. Confirm with Yes to remove the subdomain.
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