qt_themes: remove unknown qss property from dark theme

Qdarkstyle's qss file uses an overflow property.
According to `https://doc.qt.io/qt-5/stylesheet-reference.html`,
the property `overflow` doesn't exist, which leads to a warning message
in the console.

dist/qt_themes/qdarkstyle/style.qss

@@ -673,10 +673,6 @@ QTabWidget::pane {
     border-bottom-left-radius: 2px;
 }
 
-QTabWidget::tab-bar {
-    overflow: visible;
-}
-
 QTabBar {
     qproperty-drawBase: 0;
     border-radius: 3px;