Auto merge of #111134 - GilShoshan94:remove-send-bound-on-error, r=dtolnay

Remove unnecessary Send bound

Hi,

While working on a [PR on Tokio](tokio-rs/tokio#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 rust-lang/rust#23541).

In the meantime, the `Send` bound `Error` got removed (see rust-lang/rust#21312 and rust-lang/rust#23799).

So here a PR to removed this bound for `SendError<T>`, `TrySendError<T>` and `SendTimeoutError<T>`.

Author: bors