A while back, a number of folks had a similar issue, an Office Add-in named Wunderlist was removed from the Add-in Store, leaving the installation orphaned, it couldn't be removed as it no longer appeared in My Add-ins. Here's a solution for that app, which may well work for you if you change the name to the app you want to remove:
Look for the Manifests Subfolder in %AppData%\Local\Microsoft\Office\16.0\Wef
Locate the file that contains:
<ProviderName>Wunderlist</ProviderName>
Copy that file somewhere you can easily access it and add .xml to the filename
In Outlook go to File -> Manage Add-Ins and open the Web-Addins site
Click on My add-ins -> Custom add-ins -> + Add a custom add-in -> Add from file ...
Select the above renamed xml file and install it
Now you have Wunderlist installed under Custom add-ins.
Finally, click on the 3 dots -> Remove