Remove pallet::getter usage from the bounties and child-bounties pallets (#4392)
As per #3326, removes pallet::getter usage from the bounties and
child-bounties pallets. The syntax `StorageItem::<T, I>::get()` should
be used instead.
Changes to one pallet involved changes in the other, so I figured it'd
be best to combine these two.
cc @muraca
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Co-authored-by: Bastian Köcher <[email protected]>