Remove the need for setting web_sys_unstable_apis (#5000)

* No longer required since https://github.com/emilk/egui/pull/4980

And despite some outdated comments, wgpu/WebGPU doesn't need it either

crates/eframe/src/web/app_runner.rs

@@ -276,14 +276,10 @@ impl AppRunner {
             super::open_url(&open.url, open.new_tab);
         }
 
-        #[cfg(web_sys_unstable_apis)]
         if !copied_text.is_empty() {
             super::set_clipboard_text(&copied_text);
         }
 
-        #[cfg(not(web_sys_unstable_apis))]
-        let _ = copied_text;
-
         if self.has_focus() {
             // The eframe app has focus.
             if ime.is_some() {