It is deep within a counterbored fork leg so slicing a slot in it may not work. I'd get a good drill bit a hair larger than 8mm or somewhere around 5/16 to 21/64, hopefully cobalt would work. Drill it deep enough past the bolt head and the head will break off of the threaded stud inside. That piece leftover shold spin right out with a pick or a drill in reverse. Sounds like they used the wrong hex key.....
All our government does is distract us while they steal from us, misspend our tax $ and ruin our country