You DO NOT want to remove the bracket from the frame. The "braze-on" style front derailleur is bolted to the bracket with a single allen head bolt and removal of that bolt frees the derailleur for replacement with what ever "braze-on" style new derailleur you want to use.
Gonzo Bob is correct that the term braze-on originally applied to steel frames but lingers on even for materials where the mounting bracket is welded, rivited or glued in place.
The downside to these brackets, however they are attached, is that they can limit adjustability of the front derailleur and may limit your choice of chainring configuration. They are essential on non-round seat tubes where a clamp cannot be used.