Remove unused and confusing CTransaction constructor

src/test/fuzz/transaction.cpp

@@ -42,7 +42,7 @@ void test_one_input(const std::vector<uint8_t>& buffer)
             return CTransaction(deserialize, ds);
         } catch (const std::ios_base::failure&) {
             valid_tx = false;
-            return CTransaction();
+            return CTransaction{CMutableTransaction{}};
         }
     }();
     bool valid_mutable_tx = true;