To remove a custom unit label simply navigate to Settings > Custom Labels and find the Unit Labels subsection under the Products section. Select your default language (if not already selected) and find the Remove button.
Click Remove for the label(s) you wish to remove and click Save Changes when done. If you don't see a Remove button then you are not on your default language. Custom unit labels can only be translated on non-default languages and cannot be created or deleted on them.