If you'd like to remove the Split Test Automation workflow action from your HubSpot portal, please follow the below steps.
Navigate to the settings page in HubSpot by clicking on the gear icon in the top right. Underneath "Account Setup", you'll see a drop down for "Integrations" and then "Connected Apps". Navigate to "Connected Apps". You can then search for "Split Test Automation" in the search bar. Once you have located the app, click on "Actions" and "Uninstall". This will uninstall the workflow action from your HubSpot portal entirely.
One thing to be aware of is if you have setup any automated workflows using the split test automation tool (for example, testing the long term success of automated flows), you'll want to turn the workflow off and delete the action.