Make primary clipboard Linux only (#11843)

I guess only Linux supports the primary clipboard.

Release Notes:

- N/A

crates/gpui/src/platform/windows/platform.rs

@@ -625,8 +625,6 @@ impl Platform for WindowsPlatform {
         false
     }
 
-    fn write_to_primary(&self, _item: ClipboardItem) {}
-
     fn write_to_clipboard(&self, item: ClipboardItem) {
         if item.text.len() > 0 {
             let mut ctx = ClipboardContext::new().unwrap();
@@ -634,10 +632,6 @@ impl Platform for WindowsPlatform {
         }
     }
 
-    fn read_from_primary(&self) -> Option<ClipboardItem> {
-        None
-    }
-
     fn read_from_clipboard(&self) -> Option<ClipboardItem> {
         let mut ctx = ClipboardContext::new().unwrap();
         let content = ctx.get_contents().ok()?;