How do I completely remove Google Chrome apps (e.g. Postman) on Ubuntu 18.04?

Hover on any extension and click manage extensions and then search postman and choose remove.

This will remove web app postman from Chrome.

edit 1: reply to comment.

If you can't see chrome extension, then click on any extension, then the extension menu opens where you can search for Postman, then click remove.

I don't know if you manage to solve this, I had the exact same problem: I looked for postman references with both locate and find commands and removed all of them but the application was still there... Finally, I found it:

It was in ~/.local/share/applications, where you can find desktop files for different chrome extensions (with weird names). The one for Postman is: chrome-fhbjgbiflinjbdggehcddcbncdddomop-Profile_X.desktop, where the X depends on your profiles on chrome and for which of them you have the app installed.

On my machine/in my Chrome, the Postman Chrome app was also not listed as an extension.

But then I switched the Chrome user to the one, I've used before, the Postman Chrome app appeared as extension and could be removed with a mouse click.

So maybe you've simply installed under another user, just like me...:)