Remove dead code

Since the Windows nativestyle SpinBox never uses a NativeStyle.SpinBox
as a background, there is no need in having the bool __nativeBackground
property (the background is always an Item).

We can then remove/simplify all code that uses the __nativeBackground
property, since it was already always false before this change.

Also removes this warning:

  SpinBox.qml:15:5: Unable to assign [undefined] to int

Pick-to: 6.4 6.2
Change-Id: I2b9e44617884ba4523fbe17417aaccc432c1cb50
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/quickcontrols2/windows/SpinBox.qml

@@ -8,12 +8,9 @@ import QtQuick.NativeStyle as NativeStyle
 T.SpinBox {
     id: control
 
-    property bool __nativeBackground: background instanceof NativeStyle.StyleItem
     property bool nativeIndicators: up.indicator.hasOwnProperty("_qt_default")
                                     && down.indicator.hasOwnProperty("_qt_default")
 
-    font.pixelSize: __nativeBackground ? background.styleFont(control).pixelSize : undefined
-
     implicitWidth: Math.max(contentItem.implicitWidth + leftInset + rightInset,
                             90 /* minimum */ )
     implicitHeight: Math.max(contentItem.implicitHeight, up.implicitIndicatorHeight + down.implicitIndicatorHeight)
@@ -21,10 +18,10 @@ T.SpinBox {
 
     spacing: 2
 
-    leftPadding: __nativeBackground ? background.contentPadding.left: 0
-    topPadding: __nativeBackground ? background.contentPadding.top: 0
-    rightPadding: (__nativeBackground ? background.contentPadding.right : 0) + rightInset
-    bottomPadding: __nativeBackground ? background.contentPadding.bottom: 0
+    leftPadding: 0
+    topPadding: 0
+    rightPadding: rightInset
+    bottomPadding: 0
 
     validator: IntValidator {
         locale: control.locale.name