Deprecate prepareBeaconProposer, consolidate on registerValidator.

I truly believe prepareBeaconProposer is strictly more cumbersome than registerValidator.

1) It's vulnerable to [attack](https://www.symphonious.net/2022/04/09/exploring-eth2-stealing-inclusion-fees-from-public-beacon-nodes/)
2) Validator Clients have inherent knowledge of the private keys of their attached Validators, but not the indices, so the first thing many of them do is request, by public key, the index of each associated validator. This can [lead to problems](https://github.com/status-im/nimbus-eth2/issues/4980) at worst, but even with the kinks get ironed out, it represents unnecessary overhead during validator client startup.

Now that [builder_boost_factor](https://github.com/ethereum/beacon-APIs/pull/386) is present, control over whether to use a builder block or a paired node block is satisfied at the time produceBlockV3 is called, and consolidating prepareBeaconProposer and registerValidator will not leave the BN wondering which block to return to each attached validator client.