Prune fork choice equivocating indices

[michaelsproul]

Description

We added tracking of equivocating indices to fork choice in #3371, but no mechanism to prune the set of equivocators.

The spec states that we should maintain the set of equivocating indices since "at least finalization", but I think this is perhaps slightly too short, as validators remain active and capable of attesting until their exit_epoch. We could prune all validators whose exit_epoch <= finalized_epoch.