Remove unused white rectangle causing drawing issues

The spinbox has a white rectangle which overlaps the frames with some
styles (eg. oxygen). However the Rectangle itself serves no real purpose
and so it's removed.

Change-Id: I37a6862dfd8ac21fb24b54261fa7d7ab54aa23c7
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

src/controls/Styles/Desktop/SpinBoxStyle.qml

@@ -88,11 +88,6 @@ Style {
         Item {
             id: edit
             anchors.fill: parent
-            Rectangle {
-                color: "white"
-                anchors.fill: parent
-                anchors.margins: -1
-            }
             FocusFrame {
                 anchors.fill: parent
                 focusMargin:-6