Make primary clipboard Linux only (#11843)

I guess only Linux supports the primary clipboard.

Release Notes:

- N/A

crates/gpui/src/platform/mac/platform.rs

@@ -817,8 +817,6 @@ impl Platform for MacPlatform {
         }
     }
 
-    fn write_to_primary(&self, _item: ClipboardItem) {}
-
     fn write_to_clipboard(&self, item: ClipboardItem) {
         let state = self.0.lock();
         unsafe {
@@ -856,10 +854,6 @@ impl Platform for MacPlatform {
         }
     }
 
-    fn read_from_primary(&self) -> Option<ClipboardItem> {
-        None
-    }
-
     fn read_from_clipboard(&self) -> Option<ClipboardItem> {
         let state = self.0.lock();
         unsafe {