GDExtension: Don't deprecate old method of getting script category

[dsnopek]

This is a follow up to PR #78995

That PR allows GDExtensions to take control over the class category used for script instances (when you're adding a scripting language from a GDExtension). However, in many (most?) cases, GDExtensions don't need this extra control and the old, default method is just fine.

So, rather than deprecating the old behavior, I think this should just be a case where there's two ways to handle this: (1) provide a get_class_category_func if you want the extra control, or (2) provide nullptr if the default behavior is fine.

This PR just removes the #ifndef DISABLE_DEPRECATED that wraps the old behavior, and adds a comment to the gdextension_interface.h.

[Bromeon]

Makes sense to have this optional!

I was at first confused by "don't deprecate the old method" because GDExtensionScriptInstanceInfo (v1) still remains deprecated after this PR. As I understood, you re-introduce allowing nullability here.

[dsnopek]

I was at first confused by "don't deprecate the old method" because GDExtensionScriptInstanceInfo (v1) still remains deprecated after this PR. As I understood, you re-introduce allowing nullability here.

Ah, yes, sorry for the confusion. GDExtensionScriptInstanceInfo is still deprecated, it's just allowing get_class_category_func to be null "officially", not only to support the deprecated struct.

[akien-mga]

Thanks!