As I said in the previous comments, I wouldn't suggest doing that. It's not working just because we're lacking a definition. We could make it work as soon as we have a definition (we could do it now, but it doesn't make sense because it may change after further discussions).
If we remove the function, we are actually removing everything related to register the permission, including all the IPC between ep and bp, all the related functions in extension part, the XW extension interfaces, the delegate interfaces, ... It will make this PR very incomplete, it can't even compile if we register a function in external extensions.