When you see furniture or other large objects left in the park, call 311 to have city workers remove it. It's also easy to use 311 online, if you can figure out how to click your way through. For something like this chair, you'd select Parks on the main page, then Neighborhood Services, then Trash or Dumping.
The online form requires a search for location, and GGP Panhandle is how the park is listed. Try to remember the nearest cross-streets so you can include that when you describe the location.
Don't assume somebody else has already reported it. In my experience, they probably haven't.