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 <git@kchr.de>