Remove validators related settings from dBFT configuration

[AnnaShaleva]

Once Governance contract is properly integrated, we may safely remove all validators related settings from dBFT engine configuration (and from dbft section of genesis.json config files). Both validators count and a set of validators itself are available via Governance contract API.

Depends on #14. Ref. 273c95c.

[txhsl]

It seems hard to directly remove, the consensus engine is created before blockchain initialization.

go-ethereum/eth/backend.go

Line 155 in a8bf2ee

engine, err := ethconfig.CreateConsensusEngine(chainConfig, chainDb)

go-ethereum/eth/backend.go

Line 224 in a8bf2ee

eth.blockchain, err = core.NewBlockChain(chainDb, cacheConfig, config.Genesis, &overrides, eth.engine, vmConfig, eth.shouldPreserve, &config.TransactionHistory)

It's not cool to read alloc data as consensus configs, I think.