Take the whole upper arm off and just cut it off and put the arm on the bench and try to drive the remainder of the bolt out with a drift pin.
You might need to press the thing out but you could center punch the bolt then drill it out starting with a 1/4" bit and stepping it up.
These are hardened bolts so that isn't easy to do. Driving out is my first choice. Plan B is to press it out.
...oh and while it is off, determine why the thing seized up in there to begin with.
