Summary

For different reasons, admins may be required to remove the "View article in new window" link from the KB view in the customer portal.
One of them is to conceal the Confluence URL from the end-user.

Workaround

As per Atlassian Support Offerings customizations are not part of the support scope.

Removing this option would require changes in Jira's code templates, which is strongly not recommended and we cannot guarantee its consequences, however, If you still wish to change it, I'd advise testing it on your staging environment first

You can try hiding the button if it's causing a major impact. The text of the button should be located at <JIRA_HOME>/plugins/installed-plugins/jira-servicedesk-x.x.x-REL-0001.jar/i18n/sd-i18n.properties:

sd.kb.fallback.link.text=View article in <a href="{0}" target="_blank">new window {1}</a>

You may try modifying or removing the text, but note that this is outside of support.
The following articles also provide steps on how to extract the jar file to inspect or modify its content:

• Jira Service Management Internationalization