feat: drop heuropt-plot companion crate

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.

Cargo.toml

@@ -1,6 +1,3 @@
-[workspace]
-members = [".", "heuropt-plot"]
-
 [package]
 name = "heuropt"
 version = "0.8.0"
@@ -31,7 +28,6 @@ serde = { version = "1", features = ["derive"], optional = true }
 
 [dev-dependencies]
 gungraun = "0.18"
-heuropt-plot = { path = "heuropt-plot" }
 proptest = "1"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "time"] }