Remove ForkService in validator

[dapplion]

Motivation

Reverts #2574 which reverted #2529

The validator has all the necessary fork info already. A routine on start ensure that the validator and beacon node config are equal

lodestar/packages/validator/src/services/validatorStore.ts

Lines 238 to 244 in 13e8538

	private async getDomain(domainType: DomainType, epoch: Epoch): Promise<Buffer> {
	// We don't fetch the Fork from the beacon node dynamically.
	// The Fork object should not change during the runtime of the validator. When a new planned fork happens
	// node operators would have to update the validator client software at least once.
	// If we wanted to have long-term independent validator client we can review this approach.
	// On start-up the validator client fetches the full config from the beacon node and ensures they match.
	const forkVersion = this.config.getForkVersion(computeStartSlotAtEpoch(this.config, epoch));