rpc: return effective gas price from receipt

This will give us the correct result for fee currency txs. The old
approach was necessary when the code was written because receipts did
not contain the effective gas price back then.
This change could be upstreamed, if it works as expected.

internal/ethapi/api.go

@@ -1648,9 +1648,8 @@ func newRPCTransaction(tx *types.Transaction, blockHash common.Hash, blockNumber
 		result.GasFeeCap = (*hexutil.Big)(tx.GasFeeCap())
 		result.GasTipCap = (*hexutil.Big)(tx.GasTipCap())
 		// if the transaction has been mined, compute the effective gas price
-		if baseFee != nil && blockHash != (common.Hash{}) {
-			// price = min(gasTipCap + baseFee, gasFeeCap)
-			result.GasPrice = (*hexutil.Big)(effectiveGasPrice(tx, baseFee))
+		if receipt != nil {
+			result.GasPrice = (*hexutil.Big)(receipt.EffectiveGasPrice)
 		} else {
 			result.GasPrice = (*hexutil.Big)(tx.GasFeeCap())
 		}