notification-manager - remove promise listener

seems chrome changed their API?
MDN suggests that a Promise should be returned but getting `undefined`
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/windows/create

Chrome docs suggest its a callback API lolwut
https://developer.chrome.com/extensions/windows#method-create

app/scripts/lib/notification-manager.js

@@ -24,9 +24,6 @@ class NotificationManager {
           width,
           height,
         })
-        .catch((reason) => {
-          log.error('failed to create poupup', reason)
-        })
       }
     })
   }