iOS: Eliminate unguarded-availability opt-out

shell/platform/darwin/ios/BUILD.gn

@@ -199,12 +199,8 @@ shared_library("ios_test_flutter") {
   testonly = true
   visibility = [ "*" ]
 
-  # Xcode 15 beta has a bug where iOS 17 API usage is not guarded.
-  # This bug results engine build failure since the engine treats warnings as errors.
-  # The `-Wno-unguarded-availability-new` can be removed when the Xcode bug is fixed.
-  # See details in https://github.com/flutter/flutter/issues/128958.
   cflags_objc = flutter_cflags_objc
-  cflags_objcc = flutter_cflags_objcc + [ "-Wno-unguarded-availability-new" ]
+  cflags_objcc = flutter_cflags_objcc
   cflags = [
     "-fvisibility=default",
     "-F$platform_frameworks_path",