Bug 2049856 - Remove old viaduct backend code [ci full]

[bendk]

• Removed the old backend and backend::ffi modules
• Renamed new_backend to backend.

Changed the default timeout for viaduct::Client to 7 seconds for iOS and 10 seconds otherwise. This matches the behavior of we had for Request::send(). Should we make these longer?

Also: removed viaduct-reqwest which was no longer used. Removed the rusqlite::bundled feature from ads-client. It's not set by other crates and this fixed a build issue for me.

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[bendk]

From markh in an earlier version:

I don't think these make sense - I've lost some of the history, but I don't think the very first cuts of viaduct had such short timeouts. The problem is that none of us will ever see a problem as we are on good networks, but things just randomly break in the field - ie, the 10s sync timeout was introduced (I'm quite sure we never started with that timeout), but it took some time for us to finally notice in user-generated bug reports that it was a problem.
I'd rather invert this - have sane defaults and individual clients who really do need insanely short timeouts get to override?

[bendk]

Thanks for catching this, I bumped the timeout back up to 60s.

I got confused because this was the defaults for the old backend that I was removing: https://github.com/mozilla/application-services/pull/7449/changes#diff-f05535f2df59d33408a4bec415bf621aa648501e7f6f5279424e7d74684636c7L39. However, I guess that wasn't actually taking effect because all platforms are using the new backend now, which is why we can open this PR in the first place.

The thing I still don't understand why we weren't seeing timeouts before we switched to the new backend. With the old backend the defaults did seem to be 7s and 10s and I don't see how they were overridden.