Hmm, I believe the Antikythera Mechanism (https://en.wikipedia.org/wiki/Antikythera_mechanism
) may be in better shape than that calliper, and it was underwater for two thousand years...
It's the pads locked onto the disc because the calliper and pistons have rusted up.
If you *can* start by removing the calliper itself (not the frame): there's nothing there to salvage so you can cut the brake line to release pressure; then remove the rubber covers top and bottom of the calliper (visible e.g. in the last image) and remove the slider bolts - a hex bolt as I recall. With those removed you will probably find the sliders have set in place but a suitable persuasion tool (https://www.canford.co.uk/EARTH-SPIKE-FITTING-TOOL
) should get things moving - you want to move the calliper and brakes outwards and off the disk.
If that won't work, you're going to need a long bar for the leverage and a 19mm socket - ideally, hex not twelve-point for strength - on at least a half-inch drive. Use that on the two bolts that hold the calliper frame and then refer to the persuasion tool above.