Simplify livekit config so that cargo check Just Works (#21661)

Supersedes https://github.com/zed-industries/zed/pull/21653

This enables us to use `cargo test -p workspace` on macOS and Linux.

Note that the line diffs in `shared_screen.rs` are spurious, I just
re-ordered the `macos` and `cross-platform` modules to match the order
in the call crate.

Release Notes:

- N/A

crates/workspace/Cargo.toml

@@ -24,8 +24,6 @@ test-support = [
     "gpui/test-support",
     "fs/test-support",
 ]
-livekit-macos = ["call/livekit-macos"]
-livekit-cross-platform = ["call/livekit-cross-platform"]
 
 [dependencies]
 anyhow.workspace = true