Remove default package.readme metadata from Cargo.toml

Since cargo 1.46.0, README.md is recognized by default.

Author: dtolnay

Cargo.toml

@@ -9,7 +9,6 @@ documentation = "https://docs.rs/proc-macro2"
 edition = "2018"
 keywords = ["macros", "syn"]
 license = "MIT OR Apache-2.0"
-readme = "README.md"
 repository = "https://github.com/dtolnay/proc-macro2"
 rust-version = "1.31"