That's tig welded on, not brazed.
I'd be very careful with a grinder, as they can put a LOT of heat into a very thin tube very quickly, especially a flap disc - I'd 100% finish with a hand file.. In fact, I'd do the whole thing with hand tools - shouldn't take more than 5 mins TBH.
Hacksaw the mount off parallel with the tube (you can see that you only need to remove a tiny bit of metal above and below the mount) and finish with a file.