cmd/utils: validate pre-existing genesis in --dev mode (#28468)

geth --dev can be used with an existing data directory and genesis block. Since
dev mode only works with PoS, we need to verify that the merge has happened.

Co-authored-by: Felix Lange <fjl@twurst.com>

eth/catalyst/simulated_beacon.go

@@ -82,10 +82,6 @@ type SimulatedBeacon struct {
 }
 
 func NewSimulatedBeacon(period uint64, eth *eth.Ethereum) (*SimulatedBeacon, error) {
-	chainConfig := eth.APIBackend.ChainConfig()
-	if !chainConfig.IsDevMode {
-		return nil, errors.New("incompatible pre-existing chain configuration")
-	}
 	block := eth.BlockChain().CurrentBlock()
 	current := engine.ForkchoiceStateV1{
 		HeadBlockHash:      block.Hash(),