Actually, those are removable. What's holding them on is double-sided adhesive paper(or something similar). You can take them off if you get a hair-dryer, warm up the backplate and carefully pull the plate away from the board. After they're off, you should be able to fit a wide variety of heatsinks. However, looking at the hole spacing you should be able to mount a good number of heatsinks that have their own backplates. Just use the existing ones. The screw threads are standard so are very likely to be compatible.