QStylePrivate: remove unused member layoutSpacingIndex

The member layoutSpacingIndex is not used anywhere so it can be removed.

Change-Id: I0d8bc3b0ce8cf7ab26c3fd11977cfc9bffc42106
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

src/widgets/styles/qstyle_p.h

@@ -29,12 +29,8 @@ class QStylePrivate: public QObjectPrivate
 {
     Q_DECLARE_PUBLIC(QStyle)
 public:
-    inline QStylePrivate()
-        : layoutSpacingIndex(-1), proxyStyle(nullptr) {}
-
     static bool useFullScreenForPopup();
 
-    mutable int layoutSpacingIndex;
     QStyle *proxyStyle;
     QString name;
 };