mining: always returns .coinbasevalue in getblocktemplate

Although the BIPs specify that coinbasetxn and coinbasevalue are
mutually exclusive, both the latest bitcoind (22.0.0) and lbrycrd
(0.17.3) return .coinbasevalue regardeless if 'coinbasetxn' is
specified in the capabilities.

We'll make lbcd behave the same for compatibility.

rpcserver.go

@@ -2368,7 +2368,6 @@ func (state *gbtWorkState) blockTemplateResult(useCoinbaseValue bool, submitOld
 
 	if useCoinbaseValue {
 		reply.CoinbaseAux = gbtCoinbaseAux
-		reply.CoinbaseValue = &msgBlock.Transactions[0].TxOut[0].Value
 	} else {
 		// Ensure the template has a valid payment address associated
 		// with it when a full coinbase is requested.
@@ -2401,6 +2400,9 @@ func (state *gbtWorkState) blockTemplateResult(useCoinbaseValue bool, submitOld
 		reply.CoinbaseTxn = &resultTx
 	}
 
+	// Return coinbasevalue anyway as lbrycrd and bitcoind do.
+	reply.CoinbaseValue = &msgBlock.Transactions[0].TxOut[0].Value
+
 	return &reply, nil
 }