When I access to piwik on HTTP by admin user, following message was shown.
“You are currently using Piwik over unsecure HTTP, which can be risky. We recommend you set up Piwik to use SSL (HTTPS) for improved security. Read this to learn more.”
No, you can’t hide it, unless switching to https. But this message will only be shown to super users, so logging in with an account with user or admin rights wouldn’t show it.