Remove `pallet::getter` usage from authority-discovery pallet (#4091)
As per #3326, removes pallet::getter usage from the pallet authority-discovery. The syntax `StorageItem::<T, I>::get()` should be used instead. cc @muraca --------- Co-authored-by: Liam Aharon <[email protected]> Co-authored-by: Bastian Köcher <[email protected]>