Make primary clipboard Linux only (#11843)

I guess only Linux supports the primary clipboard.

Release Notes:

- N/A

crates/editor/src/element.rs

@@ -530,6 +530,7 @@ impl EditorElement {
         cx.stop_propagation();
     }
 
+    #[cfg(target_os = "linux")]
     fn mouse_middle_down(
         editor: &mut Editor,
         event: &MouseDownEvent,
@@ -3240,6 +3241,7 @@ impl EditorElement {
                         MouseButton::Right => editor.update(cx, |editor, cx| {
                             Self::mouse_right_down(editor, event, &position_map, &text_hitbox, cx);
                         }),
+                        #[cfg(target_os = "linux")]
                         MouseButton::Middle => editor.update(cx, |editor, cx| {
                             Self::mouse_middle_down(editor, event, &position_map, &text_hitbox, cx);
                         }),