[Cleanup] Remove un-used setTxBuilder in sapling_operation

The transaction builder should be initialized and set only when the
sapling_operation is constructed.

src/sapling/sapling_operation.h

@@ -99,7 +99,6 @@ class SaplingOperation {
     SaplingOperation* setRecipients(std::vector<SendManyRecipient>& vec) { recipients = std::move(vec); return this; };
     SaplingOperation* setFee(CAmount _fee) { fee = _fee; return this; }
     SaplingOperation* setMinDepth(int _mindepth) { assert(_mindepth >= 0); mindepth = _mindepth; return this; }
-    SaplingOperation* setTxBuilder(TransactionBuilder& builder) { txBuilder = builder; return this; }
     SaplingOperation* setTransparentKeyChange(CReserveKey* reserveKey) { tkeyChange = reserveKey; return this; }
     SaplingOperation* setCoinControl(const CCoinControl* _coinControl) { coinControl = _coinControl; return this; }