Auto merge of #25294 - Darkspirit:h2, r=jdm

Let hyper automatically set the host header if needed

Google's gws web server did not expect to receive an unneeded Host header via HTTP/2, therefore it responded with 400 Bad Request over a working HTTP/2 connection.

https://tools.ietf.org/html/rfc7540#page-55
> Clients that generate HTTP/2 requests directly SHOULD use the ":authority" pseudo-header field instead of the Host header field.

It's hyper's job to take care of this for the HTTP/1 case, therefore we can remove old code.
When calling [Client::builder()](https://github.com/servo/servo/blob/1974c875a1fb12103a6916c58ed2cbef8c3e32a2/components/net/connector.rs#L116-L119) we do not disable hyper's default [set_host](https://github.com/hyperium/hyper/blob/4b6099c7aa558e6b1fda146ce6179cb0c67858d7/src/client/mod.rs#L1019-L1024) config option, therefore hyper [automatically adds a Host header for non-HTTP/2 connections](https://github.com/hyperium/hyper/blob/4b6099c7aa558e6b1fda146ce6179cb0c67858d7/src/client/mod.rs#L289-L292)  based on the [URI](https://github.com/servo/servo/blob/3f663d7ab216a841e6250b5b10ce64d34caff97c/components/net/http_loader.rs#L418-L421).

r? @jdm

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