If you no longer want to use the Klarna On-site messaging app for Shopify in your store, in order to remove all inserted code for your store, first disconnect the account from the app Settings page. (Do not go back to the app Dashboard or it will automatically reconnect your account.) Then delete the app from your Shopify store -> Apps page, per screen shot below. All code will thus be removed from your store.
If you already removed the Klarna On-site messaging app, but the KlarnaThemeGlobals code was not removed from your theme, you can manually remove it from the theme.liquid file in your store admin from Online Store -> Actions -> Edit code