Remove bounds from `PrevalidateAttests` struct definition (#2886)

Removing some bounds as it came up in
https://github.com/paritytech/polkadot-sdk/pull/2726 while still keeping
`Send + Sync` capabilities.

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