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>

params/config.go

@@ -180,7 +180,6 @@ var (
 		ShanghaiTime:                  newUint64(0),
 		TerminalTotalDifficulty:       big.NewInt(0),
 		TerminalTotalDifficultyPassed: true,
-		IsDevMode:                     true,
 	}
 
 	// AllCliqueProtocolChanges contains every protocol change (EIPs) introduced
@@ -329,9 +328,8 @@ type ChainConfig struct {
 	TerminalTotalDifficultyPassed bool `json:"terminalTotalDifficultyPassed,omitempty"`
 
 	// Various consensus engines
-	Ethash    *EthashConfig `json:"ethash,omitempty"`
-	Clique    *CliqueConfig `json:"clique,omitempty"`
-	IsDevMode bool          `json:"isDev,omitempty"`
+	Ethash *EthashConfig `json:"ethash,omitempty"`
+	Clique *CliqueConfig `json:"clique,omitempty"`
 }
 
 // EthashConfig is the consensus engine configs for proof-of-work based sealing.