Make CosmicTextSystem Linux-only (#14728)

Since `WindowsDispatcher` requires a minimum Windows version of Windows
10 Fall Creators Update (10.0.16299), and the `alacritty_terminal`
dependency relies on conPTY, an API introduced in the same version,
additionally, `DirectWriteTextSystem` also relies on Windows 10 Fall
Creators Update (10.0.16299), so it seems reasonable to make
`CosmicTextSystem` Linux-only. And we can use `DirectWriteTextSystem` on
the Windows platform exclusively. I hope this approach makes sense.

Release Notes:

- N/A

crates/gpui/src/platform.rs

@@ -4,9 +4,6 @@
 mod app_menu;
 mod keystroke;
 
-#[cfg(not(target_os = "macos"))]
-mod cosmic_text;
-
 #[cfg(target_os = "linux")]
 mod linux;
 
@@ -51,8 +48,6 @@ use uuid::Uuid;
 pub use app_menu::*;
 pub use keystroke::*;
 
-#[cfg(not(target_os = "macos"))]
-pub(crate) use cosmic_text::*;
 #[cfg(target_os = "linux")]
 pub(crate) use linux::*;
 #[cfg(target_os = "macos")]
@@ -105,7 +100,6 @@ pub fn guess_compositor() -> &'static str {
     }
 }
 
-// todo("windows")
 #[cfg(target_os = "windows")]
 pub(crate) fn current_platform(_headless: bool) -> Rc<dyn Platform> {
     Rc::new(WindowsPlatform::new())
@@ -413,8 +407,6 @@ pub(crate) trait PlatformTextSystem: Send + Sync {
         raster_bounds: Bounds<DevicePixels>,
     ) -> Result<(Size<DevicePixels>, Vec<u8>)>;
     fn layout_line(&self, text: &str, font_size: Pixels, runs: &[FontRun]) -> LineLayout;
-    #[cfg(target_os = "windows")]
-    fn destroy(&self);
 }
 
 #[derive(PartialEq, Eq, Hash, Clone)]