Set Qt::HighDpiScaleFactorRoundingPolicy::PassThrough also on Unix

Previously this was Windows-only, the reasons are described in

  https://bugs.kde.org/show_bug.cgi?id=416078

However, one and a half years later I don't see any negative effects
when setting this option,  (In fact, I don't see any effects at all),
both for X11 and Wayland.  Let's therefore enable PassThrough for
Unix as well, in order to reduce the amount of platform-specific code.

shell/main.cpp

@@ -35,11 +35,8 @@ int main(int argc, char **argv)
 
     /**
      * allow fractional scaling
-     * we only activate this on Windows, it seems to creates problems on unices
-     * (and there the fractional scaling with the QT_... env vars as set by KScreen works)
-     * see bug 416078
      */
-#if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0) && defined(Q_OS_WIN)
+#if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0)
     QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
 #endif