You only need to remove the head of the bolt and some of the threaded section thats inserted into this bit of the clamp that isn’t threaded, to enable you to get the clamp past the bit thats left, not drill the whole thing out.
So start with something like a 3mm drill bit to this depth, you can use some tape around the bit as a depth marker. Then do the same with incrementally bigger bits, i.e. 3.5, 4, 4.5 and finally, 5mm, 5.5mm may well be needed to separate the head from the threaded section. You may find you don’t need to go in 0.5mm steps, so see how you go.
Once you’ve done this, you should be able to place a lever like a flat blade screwdriver between the clamp surfaces and gently prise them apart and over the stub remaining.
You’ll obviously find it easier to work on if you rotate the bars, so that the bolt head is pointing up.