Administrator installed add-in
At the time of writing it was not possible to delete an administrator installed add-in using Microsoft's new admin center. You need to set the "Try the new admin center" toggle to off.
From the admin center menu choose Settings, Services & add-ins, click on Appogee Leave and then click the Delete Add-in option as shown below:
User installed add-in
Should you wish to remove the Appogee Leave add-in for Microsoft Outlook simply open the settings cog at the top right of your Outlook screen, navigate to Add-ins and look at My Add-ins. When you locate Appogee Leave click the 3 dot menu to the right of Added to find the Remove option. If you want to prevent new users from getting the add-in you will want to go to the Exchange Admin Center under Organization choose Add-ins and delete the Appogee Leave add-in from there.