chore: remove doc URL from Cargo.toml (#4251)

https://doc.rust-lang.org/cargo/reference/manifest.html#the-documentation-field

> If no URL is specified in the manifest file, crates.io will
> automatically link your crate to the corresponding docs.rs page.

tokio-util/Cargo.toml

@@ -2,8 +2,6 @@
 name = "tokio-util"
 # When releasing to crates.io:
 # - Remove path dependencies
-# - Update doc url
-#   - Cargo.toml
 # - Update CHANGELOG.md.
 # - Create "tokio-util-0.6.x" git tag.
 version = "0.6.9"
@@ -13,7 +11,6 @@ authors = ["Tokio Contributors <[email protected]>"]
 license = "MIT"
 repository = "https://github.com/tokio-rs/tokio"
 homepage = "https://tokio.rs"
-documentation = "https://docs.rs/tokio-util/0.6.9/tokio_util"
 description = """
 Additional utilities for working with Tokio.
 """