worker: no need to add more transaction if not mining

It could be a very old PoW logic, which try to add more transaction
into the pending block when mining is stopped.
Mining can be stopped when:
  1.download started.
  2.manually stopped by RPC.
It is unnecessary to add more transaction into the pending block if a validator is stopped.
And updateSnapshot() is not needed as well, it is to get the pending mining snapshot.

miner/worker.go

@@ -542,41 +542,14 @@ func (w *worker) mainLoop() {
 			}
 
 		case ev := <-w.txsCh:
-			// Apply transactions to the pending state if we're not sealing
-			//
-			// Note all transactions received may not be continuous with transactions
-			// already included in the current sealing block. These transactions will
-			// be automatically eliminated.
-			if !w.isRunning() && w.current != nil {
-				start := time.Now()
-				// If block is already full, abort
-				if gp := w.current.gasPool; gp != nil && gp.Gas() < params.TxGas {
-					continue
-				}
-				txs := make(map[common.Address]types.Transactions)
-				for _, tx := range ev.Txs {
-					acc, _ := types.Sender(w.current.signer, tx)
-					txs[acc] = append(txs[acc], tx)
-				}
-				txset := types.NewTransactionsByPriceAndNonce(w.current.signer, txs, w.current.header.BaseFee)
-				tcount := w.current.tcount
-				w.commitTransactions(w.current, txset, nil, nil)
-				commitTxsTimer.UpdateSince(start)
-
-				// Only update the snapshot if any new transactions were added
-				// to the pending block
-				if tcount != w.current.tcount {
-					w.updateSnapshot(w.current)
-				}
-			} else {
-				// Special case, if the consensus engine is 0 period clique(dev mode),
-				// submit sealing work here since all empty submission will be rejected
-				// by clique. Of course the advance sealing(empty submission) is disabled.
-				if (w.chainConfig.Clique != nil && w.chainConfig.Clique.Period == 0) ||
-					(w.chainConfig.Parlia != nil && w.chainConfig.Parlia.Period == 0) {
-					w.commitWork(nil, true, time.Now().Unix())
-				}
+			// Special case, if the consensus engine is 0 period clique(dev mode),
+			// submit sealing work here since all empty submission will be rejected
+			// by clique. Of course the advance sealing(empty submission) is disabled.
+			if (w.chainConfig.Clique != nil && w.chainConfig.Clique.Period == 0) ||
+				(w.chainConfig.Parlia != nil && w.chainConfig.Parlia.Period == 0) {
+				w.commitWork(nil, true, time.Now().Unix())
 			}
+
 			atomic.AddInt32(&w.newTxs, int32(len(ev.Txs)))
 
 		// System stopped