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.

fuzz/Cargo.toml

@@ -1,7 +1,3 @@
-[workspace]
-# Detach from the parent heuropt workspace so cargo-fuzz can build
-# this crate independently without listing it as a workspace member.
-
 [package]
 name = "heuropt-fuzz"
 version = "0.0.0"