I ran into this problem several years ago. Since I have many types of medium to high speed grinding drives, Like: variable speed Dremel, high speed pneumatic grinders...
I used a pneumatic die grinder with an extension shaft that holds most of my various carbide high speed grinders, diamond coated bits, and high speed deburring bits.
With bright light angled down the tube, I experimented with different bits until I found on that cut down the ridge, but wasn't too aggressive. it took me about an hour of careful massaging to slowly work my way down the seat tube. If I recall, I ended up using one of my diamond coated bits the most. (Plus you need a steady hand because the bits will want to move off the ridge and down to the side of the ridge where the rotation/torque on the bit wants to more it off of the ridge.)
Additional note: Carbon fiber tends to be harder on the surface, then after you cut through the top surface, it cuts a lot faster, so start slowly and be patient.
It is also a must to have variable speed. With an extension shaft, it becomes very easy for the shaft to get out of balance and bounce around inside the tube, so you need to be able to slow down the RPMs if/when this happens.
