Remove ::IsStandardTx(tx, reason) alias

Apart from tests, it is only used in one place, so there is no need for
an alias.

src/test/fuzz/transaction.cpp

@@ -92,7 +92,6 @@ FUZZ_TARGET_INIT(transaction, initialize_transaction)
     (void)GetTransactionWeight(tx);
     (void)GetVirtualTransactionSize(tx);
     (void)IsFinalTx(tx, /* nBlockHeight= */ 1024, /* nBlockTime= */ 1024);
-    (void)IsStandardTx(tx, reason);
     (void)RecursiveDynamicUsage(tx);
     (void)SignalsOptInRBF(tx);