Remove `@NonNull` to avoid warning (#129472)

Otherwise android studio complains:

> Do not use @NonNull in Kotlin; the nullability is already implied by the Kotlin type MethodCall not ending with ?

![image](https://github.com/flutter/flutter/assets/5236035/0cc2e838-dbf9-409f-8fd8-d4e006f58be6)

packages/flutter_tools/templates/plugin/android-kotlin.tmpl/src/main/kotlin/androidIdentifier/pluginClass.kt.tmpl

@@ -16,12 +16,12 @@ class {{pluginClass}}: FlutterPlugin, MethodCallHandler {
   /// when the Flutter Engine is detached from the Activity
   private lateinit var channel : MethodChannel
 
-  override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
+  override fun onAttachedToEngine(flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
     channel = MethodChannel(flutterPluginBinding.binaryMessenger, "{{projectName}}")
     channel.setMethodCallHandler(this)
   }
 
-  override fun onMethodCall(@NonNull call: MethodCall, @NonNull result: Result) {
+  override fun onMethodCall(call: MethodCall, result: Result) {
     if (call.method == "getPlatformVersion") {
       result.success("Android ${android.os.Build.VERSION.RELEASE}")
     } else {
@@ -29,7 +29,7 @@ class {{pluginClass}}: FlutterPlugin, MethodCallHandler {
     }
   }
 
-  override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
+  override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
     channel.setMethodCallHandler(null)
   }
 }