Remove `surf` support

[msrd0]

surf support is outdated and surf is no longer maintained. It should be removed from this crate.

ORIGINAL ISSUE TEXT:
The surf crate has not only seen no releases in the last two years, but also the git repository has seen no commit in the last two years. It depends on outdated version like tokio 0.2 and there does not seem to be any effort to make a new releases with more up to date dependencies. However, it is the only way this crate works with async-std.

I personally am fine with tokio-only libraries and would vote to remove surf in an upcoming release. Are there any users of the surf backend of this library? Would you be able to upgrade to the reqwest backend? What are your reasons for still using surf despite the default client being reqwest for a while now?

[msrd0]

The maintainer of surf said it should be considered unmaintained: http-rs/surf#352 (comment)

[Empty2k12]

This has been open for close to a month without anyone indicating that surf is required. Renaming this ticket.

[msrd0]

Given that async-std has been deprecated as well, we can probably just remove all surf/async-std all together and only support reqwest/tokio going forward.