There maybe addon domain names you no longer need in your hosting account, you can remove them by following the instructions below on how to delete addon domain name.
Delete addon domain name via cPanel:
Step 1. Log in to the cPanel.
Step 2. Click on Addon Domains under Domains panel.
Step 3. Click on Remove icon under Actions of the domain name you’d like to delete.
Step 4. You will be asked if you are sure, click on Remove button to proceed.