Remove getter macro from pallet-insecure-randomness-collective-flip (#4839)

As per #3326, removes pallet::getter macro usage from the
pallet-insecure-randomness-collective-flip. The syntax `StorageItem::<T,
I>::get()` should be used instead.

Explicitly implements the getters that were removed as well, following
#223

Also makes the storage values public and converts some syntax to turbo

cc @muraca

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>