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/zed/Cargo.toml

@@ -127,12 +127,6 @@ welcome.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true
 
-[target.'cfg(target_os = "macos")'.dependencies]
-workspace = { workspace = true, features = ["livekit-macos"] }
-
-[target.'cfg(not(target_os = "macos"))'.dependencies]
-workspace = { workspace = true, features = ["livekit-cross-platform"] }
-
 [target.'cfg(target_os = "windows")'.dependencies]
 windows.workspace = true