Remove dead code from ApplicationBuilder

[Propaganistas]

I was looking for a way to add an additional routing configuration in bootstrap/app.php using named arguments in the withRouting() syntax. Seems like there's a $then parameter that supports a callback to be executed after default configurations. Great.

However I also noticed there's a property $additionalRoutingCallbacks defined and being iterated, but I don't see any possible way to actually populate that property.

Because $then already achieves the very same, this might be dead code.
Or if the existence of $additionalRoutingCallbacks is still applicable, then you might want to review how it can be set.

This is just a heads-up PR.

[moory-se]

This can be used in inherited classes. It's useful, but mainly would be a (very) breaking change.

[AndrewMast]

While it could potentially be a breaking change (and thus should probably be targeted to master), I have not been able to find any instances where $additionalRoutingCallbacks is used. Nothing shows up in a search engine or in a quick GitHub search.

Even in Taylor's Laracon 2024 PR that added the $additionalRoutingCallbacks property, I could find no other references besides the ones that are being removed in this PR.

[taylorotwell]

Going to just leave it for now. Thanks.