Validator registrations submitted before validator activation

[realbigsean]

Description

Brought up in Discord

This line sneakily makes sure we don't include pre-activation validators in our prepare beacon proposer requests https://github.com/sigp/lighthouse/blob/stable/validator_client/src/preparation_service.rs#L269

Because there's no validator index necessary for the equivalent validator registration message, we lose this filtering https://github.com/sigp/lighthouse/blob/stable/validator_client/src/preparation_service.rs#L293