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

graphql/graphql_test.go

@@ -376,10 +376,7 @@ func createNode(t *testing.T) *node.Node {
 
 func newGQLService(t *testing.T, stack *node.Node, gspec *core.Genesis, genBlocks int, genfunc func(i int, gen *core.BlockGen)) (*handler, []*types.Block) {
 	ethConf := &ethconfig.Config{
-		Genesis: gspec,
-		Ethash: ethash.Config{
-			PowMode: ethash.ModeFake,
-		},
+		Genesis:                 gspec,
 		NetworkId:               1337,
 		TrieCleanCache:          5,
 		TrieCleanCacheJournal:   "triecache",