It's code archaeology, but if i remember correctly the purpose of this was to generate RNA properties that have a distinct subtype, limits, etc to match the socket declaration of the node group.

We're not actually using that. Instead we generate all the different type/subtype variants in advance (e.g. NodeSocketFloatDistance, NodeSocketFloatPercentage) and use a matching one for group sockets, with limits being stored in the default_value structs (e.g. bNodeSocketValueFloat) rather than an RNA property.

I probably forgot to remove it after initially adding it and then switching to predefined RNA types. No objection from my side to clean this up. The interface_register_properties callback is only used for this RNA declaration should also be removed, although it is technically registerable and some addon could theoretically break (i.e. get an error when trying to register the callback, even though it serves no purpose).