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

eth/api.go

@@ -24,7 +24,6 @@ import (
 	"io"
 	"math/big"
 	"os"
-	"runtime"
 	"strings"
 	"time"
 
@@ -86,11 +85,8 @@ func NewMinerAPI(e *Ethereum) *MinerAPI {
 // usable by this process. If mining is already running, this method adjust the
 // number of threads allowed to use and updates the minimum price required by the
 // transaction pool.
-func (api *MinerAPI) Start(threads *int) error {
-	if threads == nil {
-		return api.e.StartMining(runtime.NumCPU())
-	}
-	return api.e.StartMining(*threads)
+func (api *MinerAPI) Start() error {
+	return api.e.StartMining()
 }
 
 // Stop terminates the miner, both at the consensus engine level as well as at