eth, cmd: remove syncTarget from eth config (#26330)

--syncTarget is a feature for development purpose in post-merge world. Previously
it's added into eth.Config. But it turns out that's a stupid idea.

- syncTarget is a block object, which is hard to be put in config file(large)
- syncTarget is just a dev feature, doesn't make too much sense to add it in config file

So I remove it from the eth config object. And it also fixes the #26328

cmd/geth/config.go

@@ -31,6 +31,7 @@ import (
 	"github.com/ethereum/go-ethereum/accounts/scwallet"
 	"github.com/ethereum/go-ethereum/accounts/usbwallet"
 	"github.com/ethereum/go-ethereum/cmd/utils"
+	"github.com/ethereum/go-ethereum/eth/downloader"
 	"github.com/ethereum/go-ethereum/eth/ethconfig"
 	"github.com/ethereum/go-ethereum/internal/ethapi"
 	"github.com/ethereum/go-ethereum/internal/flags"
@@ -165,7 +166,7 @@ func makeFullNode(ctx *cli.Context) (*node.Node, ethapi.Backend) {
 		cfg.Eth.OverrideTerminalTotalDifficultyPassed = &override
 	}
 
-	backend, _ := utils.RegisterEthService(stack, &cfg.Eth)
+	backend, eth := utils.RegisterEthService(stack, &cfg.Eth)
 
 	// Configure log filter RPC API.
 	filterSystem := utils.RegisterFilterAPI(stack, backend, &cfg.Eth)
@@ -179,6 +180,11 @@ func makeFullNode(ctx *cli.Context) (*node.Node, ethapi.Backend) {
 	if cfg.Ethstats.URL != "" {
 		utils.RegisterEthStatsService(stack, backend, cfg.Ethstats.URL)
 	}
+
+	// Configure full-sync tester service if requested
+	if ctx.IsSet(utils.SyncTargetFlag.Name) && cfg.Eth.SyncMode == downloader.FullSync {
+		utils.RegisterFullSyncTester(stack, eth, ctx.Path(utils.SyncTargetFlag.Name))
+	}
 	return stack, backend
 }