how do I remove this old track light hanger from junction box?
If you can push the machine screws up, then you can just spin the bottom half of the bracket and get at the screws that are holding it.

If you can't push them up, grab them with locking pliers (ie, visegrips) and spin them out.

If that doesn't work, use brute force to bend the bottom half of the bracket to give you access to the screws holding the top half. Then you'll be in a better position to assess the hanging machine screws.

oh, and so I can sleep well at night, please put a cap on that black wire... you never know who'll flip a switch on you at the wrong time...