[ios][tool] xcode_backend logging shouldn't rely on `exitCode`

We have this code:

https://github.com/flutter/flutter/blob/5483fef5384821748cfcc1bfc3157e1f9c0322ea/packages/flutter_tools/bin/xcode_backend.dart#L132-L139

We check `exitCode` of current process, not `result.exitCode` of the subprocess (e.g. plutil).

I think this check shouldn't be necessary, because if exitCode is not 0, then the current process is already terminated (`xcode_backend` never change its own exitCode without exiting the process).

It should be safe to just do

```
if (!verbose) {
// ...
}
```

Open packages/flutter_tools/bin/xcode_backend.dart around lines 132-139 and compare the current-process exitCode check with the referenced subprocess result. Remove the unnecessary exitCode-dependent logging condition so it only checks verbose, then verify the surrounding xcode_backend behavior.

Written by the indexing model from the issue text.