core: remove useless assignments (#29065)

Author: zhiqiangxu

core/state_transition.go

@@ -234,7 +234,7 @@ func (st *StateTransition) to() common.Address {
 
 func (st *StateTransition) buyGas() error {
 	mgval := new(big.Int).SetUint64(st.msg.GasLimit)
-	mgval = mgval.Mul(mgval, st.msg.GasPrice)
+	mgval.Mul(mgval, st.msg.GasPrice)
 	balanceCheck := new(big.Int).Set(mgval)
 	if st.msg.GasFeeCap != nil {
 		balanceCheck.SetUint64(st.msg.GasLimit)
@@ -477,7 +477,7 @@ func (st *StateTransition) refundGas(refundQuotient uint64) uint64 {
 
 	// Return ETH for remaining gas, exchanged at the original rate.
 	remaining := uint256.NewInt(st.gasRemaining)
-	remaining = remaining.Mul(remaining, uint256.MustFromBig(st.msg.GasPrice))
+	remaining.Mul(remaining, uint256.MustFromBig(st.msg.GasPrice))
 	st.state.AddBalance(st.msg.From, remaining)
 
 	// Also return remaining gas to the block gas counter so it is