To remove the SigningHub for SharePoint add-in from the "Site Contents" section: 

1. Log into your Office 365 account with SharePoint administrator credentials.

2. Click the "Site Contents" option from the Home screen.

3. Select the SigningHub add-in, click the ... option from it and then click "Remove", as marked with arrows on the below screen. A confirmation message will be displayed click OK to confirm the removing activity.

SigningHub for SharePoint will be removed from the "Site Contents" section.

After removing SigningHub for SharePoint, delete the add-in from the "Recycle Bin" of the "Site Contents" page, and then from "Second-stage Recycle Bin".