Object: Remove unused category boilerplate

We might want to re-add something like this if/when we find a good use case
for it and do the effort to categorize all objects in the API properly.

Until then, it's better to remove that boilerplate since it's not needed.

Closes #18711.

core/doc_data.h

@@ -152,7 +152,7 @@ public:
 	struct ClassDoc {
 		String name;
 		String inherits;
-		String category;
+		String category; // FIXME: Wrongly used by VisualScriptPropertySelector, should be removed.
 		String brief_description;
 		String description;
 		Vector<TutorialDoc> tutorials;