Hangouts, Google Voice, and SMS is no longer supported Notification
I had the same problem and I also wanted to avoid installing an extra app just to get rid of a notification. 

The only thing I could think of is Facebook messenger which I already use. After I downloaded it the notification disappeared and messenger was now the default app for SMS.

In case the notification doesn't disappear automatically go to:

Settings -> Apps -> (Gear Icon top right) -> Default Apps

and you should see 'SMS App' just tap on it and choose messenger(or any other app you want to try). My tablet has Android 6.0.1 so instructions may differ on other versions.

PS. Firstly I tried to revoke any SMS permisions to hangouts so I can clear it from the default app for SMS but it didn't work.