Prefer no-std url crate

[Kailai-Wang]

Context

IMO we should always prefer the no-std version of a crate over sgx-ported one.

This PR tries to replace url-sgx with a no-std rust-url crate (a forked one).

Please note in some scenarios where a IntoClientRequest is expected (e.g. tungstenite.client_tls_with_config) we are using the &str that has to include scheme. It's because Url to SocketAddress conversion is only possible under std. cc @kziemianek

We should be able to switch to the more "standard" crate once core::net is supported, see servo/rust-url#831 (comment)

It removes some unused code/import and renames the stf-task-sender a bit too - I think the path core/stf-task/sender/src/stf_task_sender.rs makes little sense when we include stf_task_sender twice

[kziemianek]

I totally agree that we should prefer no std crates over std/sgx.
I hope we will be able to switch to url from crates soon :)