Remove duplicate state in ProtoArray

[dapplion]

Issue Addressed

Part of a fork-choice tech debt clean-up #8325

#7089 (non-finalized checkpoint sync) changes the meaning of the checkpoints inside fork-choice. It turns out that we persist the justified and finalized checkpoints twice in fork-choice

1. Inside the fork-choice store
2. Inside the proto-array

There's no reason for 2. except for making the function signature of some methods smallers. It's not consistent with the rest of the crate, because in some functions we pass the external variable of time (current_slot) via args, but then read the finalized checkpoint from the internal state. Passing both variables as args makes fork-choice easier to reason about at the cost of a few extra lines.

Proposed Changes

Remove the unnecessary state (justified_checkpoint, finalized_checkpoint) inside ProtoArray, to make it easier to reason about.

[michaelsproul]

Needs a schema migration, see my comment.