From the pics I've seen it appears that this bike came with V-brakes, not canti's. Can't imagine swapping those out for discs, but it's your bike. Those type mounts are a steel stamping for the frame/fork attachment. The posts are furnace brazed to the stamping. It appeared the mounts are attached to the fork with TIG. I'll tell you'll play hell removing the post from the mount with heat without at least doing some cosmetic damage. If you intend to remove the posts but keep the mounts, cut off most of the post and file, or drill out the remainder.