sync: optimization: don't draw previous entry when validating

[arajasek]

We currently (<=nv21) load the "latest" beacon entry when validating a new block. This is needed to validate our current chained beacon.

Post-nv22, we will be using unchained randomness (see FIP-0063), and therefore do not need to look up the previous randomness when validating. We can omit this step from block validation as a sync optimization.