cmd/geth: remove the tail "," from genesis config (#17028)

remove the tail "," from genesis config,  which will cause genesis config parse error .

cmd/geth/genesis_test.go

@@ -77,7 +77,7 @@ var customGenesisTests = []struct {
 				"homesteadBlock" : 314,
 				"daoForkBlock"   : 141,
 				"daoForkSupport" : true
-			},
+			}
 		}`,
 		query:  "eth.getBlock(0).nonce",
 		result: "0x0000000000000042",