Opportunities to tidy up `raft.h`

[heidihoward]

Whilst comparing raft.h to the TLA+, I spotted a few possible opportunities to tidy up raft.h. None of these changes should alter the behavior of raft.h (though I might be wrong!).

1. new_view_idx appears to be unused
2. unused if statement https://github.com/microsoft/CCF/blob/8f6d14c6f42264f30c5a65b5c745d6586317978b/src/consensus/aft/raft.h#L1109-L1118
3. Comment out-of-sync with code https://github.com/microsoft/CCF/blob/8f6d14c6f42264f30c5a65b5c745d6586317978b/src/consensus/aft/raft_types.h#L161-L168