Removes the heuropt-plot subcrate, the visualize example that used it, and the related workspace plumbing (root [workspace] table, the [workspace] override added to fuzz/Cargo.toml to detach from it, heuropt-plot dev-dep, CHANGELOG mention). The visualization concern is better served as an independent third- party project than as a companion crate in this repo. No effect on heuropt's public API or the async work in 0.8.0.