Remove reference to non-existent README.md in wasi-tokio crate

Author: cfallin

crates/wasi-common/tokio/Cargo.toml

@@ -7,7 +7,6 @@ license = "Apache-2.0 WITH LLVM-exception"
 categories = ["wasm"]
 keywords = ["webassembly", "wasm"]
 repository = "https://github.com/bytecodealliance/wasmtime"
-readme = "README.md"
 edition = "2018"
 include = ["src/**/*", "LICENSE" ]