Remove unnecessary Send bound Hi, While working on a [PR on Tokio](https://github.com/tokio-rs/tokio/pull/5666), I took inspiration from the std channel mpsc and stumbled on a `Send` bound for a `Error` impl. Tokio's maintainer `@Darksonn` pointed out to me that `Error` used to required the `Send` bound long time ago (here https://github.com/rust-lang/rust/pull/23541). In the meantime, the `Send` bound `Error` got removed (see https://github.com/rust-lang/rust/pull/21312 and https://github.com/rust-lang/rust/pull/23799). So here a PR to removed this bound for `SendError<T>`, `TrySendError<T>` and `SendTimeoutError<T>`.