Remove deprecated fields from `EpochConfig`

Three fields have been marked as deprecated and given defaults in #15425. They cannot be removed right now, because dumped configs would be backwards-incompatible with the latest stable neard release. When the deprecation is released, these fields can be safely removed:
* `num_block_producer_seats_per_shard`
* `avg_hidden_validator_seats_per_shard`
* `num_chunk_only_producer_seats`