clang-tidy, qt: Fix modernize-use-default-member-init in headers

See https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-default-member-init.html

src/qt/transactiontablemodel.cpp

@@ -253,7 +253,6 @@ TransactionTableModel::TransactionTableModel(const PlatformStyle *_platformStyle
         QAbstractTableModel(parent),
         walletModel(parent),
         priv(new TransactionTablePriv(this)),
-        fProcessingQueuedTransactions(false),
         platformStyle(_platformStyle)
 {
     subscribeToCoreSignals();