qt: drop workaround for QTBUG-42503 which was fixed in Qt 5.5.0

Fixes bitcoin-core#101

Author: prusnak

src/qt/overviewpage.cpp

@@ -126,7 +126,6 @@ OverviewPage::OverviewPage(const PlatformStyle *platformStyle, QWidget *parent)
 
     // use a SingleColorIcon for the "out of sync warning" icon
     QIcon icon = platformStyle->SingleColorIcon(":/icons/warning");
-    icon.addPixmap(icon.pixmap(QSize(64,64), QIcon::Normal), QIcon::Disabled); // also set the disabled icon because we are using a disabled QPushButton to work around missing HiDPI support of QLabel (https://bugreports.qt.io/browse/QTBUG-42503)
     ui->labelTransactionsStatus->setIcon(icon);
     ui->labelWalletStatus->setIcon(icon);