Since you will not be reusing either the bolt or the hole, the easiest way to remove the broken bolt is to simply drill it out. The first step is to cut off the top of the bolt, if possible. Using a reciprocating saw (often called a Sawzall) and a metal cutting blade, simply cut through the top of the bolt as low as possible.
How do I get a broken hardened bolt out?
How do you get a sheared bolt out of a hole?
How do you remove a ruined screw?
Related Question how to remove a damaged bolt
How do you get a stripped hex bolt out of metal?
How do you remove seized stainless steel bolts?
Will a cobalt bit drill hardened steel?
Go to a hardware or home improvement store for a cobalt bit that is designed specifically for drilling through steel. You want a cobalt bit, as it's a type of high-speed steel (HSS) that has more cobalt in it and is strong enough to cut through hardened steel.