rustdoc: Switch to mainline rayon

The rustc fork of rayon integrates with Cargo's jobserver to limit the
amount of parallelism. However, rustdoc's use case is concurrent I/O,
which is not CPU-heavy, so it should be able to use mainline rayon.

See this discussion [1] for more details.

[1]: https://github.com/rust-lang/rust/issues/90227#issuecomment-952468618

Note: I chose rayon 1.3.1 so that the rayon version used elsewhere in
the workspace does not change.

Cargo.lock

@@ -4615,8 +4615,8 @@ dependencies = [
  "itertools 0.9.0",
  "minifier",
  "pulldown-cmark 0.8.0",
+ "rayon",
  "regex",
- "rustc-rayon",
  "rustdoc-json-types",
  "serde",
  "serde_json",

src/librustdoc/Cargo.toml

@@ -10,7 +10,7 @@ path = "lib.rs"
 arrayvec = { version = "0.7", default-features = false }
 pulldown-cmark = { version = "0.8", default-features = false }
 minifier = "0.0.41"
-rayon = { version = "0.3.0", package = "rustc-rayon" }
+rayon = "1.3.1"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 smallvec = "1.6.1"