Revert "Use livekit's Rust SDK instead of their swift SDK (#13343)" (#20809)

Issues found:

* audio does not work well with various set-ups using USB
* switching audio during initial join may leave the client with no audio
at all
* audio streaming is done on the main thread, beachballing certain
set-ups
* worse screenshare quality (seems that there's no dynamic scaling
anymore, compared to the Swift SDK)

This reverts commit 1235d0808e.

Release Notes:

- N/A

crates/gpui/src/platform/test.rs

@@ -7,5 +7,3 @@ pub(crate) use dispatcher::*;
 pub(crate) use display::*;
 pub(crate) use platform::*;
 pub(crate) use window::*;
-
-pub use platform::TestScreenCaptureSource;