remove pallet::getter from pallet-offences (#6027)

# Description
Part of https://github.com/paritytech/polkadot-sdk/issues/3326
Removes pallet::getter from pallet-offences from type `Reports`.
Adds a test to verify that retrieval of `Reports` still works with
storage::getter.

polkadot address: 155YyFVoMnv9BY6qxy2i5wxtveUw7WE1n5H81fL8PZKTA1Sd

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Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Dónal Murray <donal.murray@parity.io>