all: remove ethash pow, only retain shims needed for consensus and tests (#27178)

* all: remove ethash pow, only retain shims needed for consensus and tests

* all: thank you linter

* all: disallow launching Geth in legacy PoW mode

* cmd/env/internal/t8ntool: remove dangling ethash flag

ethclient/gethclient/gethclient_test.go

@@ -56,7 +56,6 @@ func newTestBackend(t *testing.T) (*node.Node, []*types.Block) {
 	}
 	// Create Ethereum Service
 	config := &ethconfig.Config{Genesis: genesis}
-	config.Ethash.PowMode = ethash.ModeFake
 	ethservice, err := eth.New(n, config)
 	if err != nil {
 		t.Fatalf("can't create new ethereum service: %v", err)