chore: remove deprecated manual worker registration for Flutter web and the non-existent lib/generated_plugin_registrant.dart gitignore item

Author: w568w

.gitignore

@@ -34,9 +34,6 @@
 lib/generated/
 **/*.g.dart
 
-# Web related
-lib/generated_plugin_registrant.dart
-
 # Symbolication related
 app.*.symbols
 

web/index.html

@@ -40,11 +40,7 @@
        application. For more information, see:
        https://developers.google.com/web/fundamentals/primers/service-workers -->
   <script>
-    if ('serviceWorker' in navigator) {
-      window.addEventListener('flutter-first-frame', function () {
-        navigator.serviceWorker.register('flutter_service_worker.js');
-      });
-    }
+    {{flutter_bootstrap_js}}
   </script>
   <script src="main.dart.js" type="application/javascript"></script>
 </body>