Don't set the role to Accessible.Button

If an item intend to expose its children, it should not set its role
to Button

Change-Id: I6b56d1354ce06f65513688ef38c507bd2881ce82
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@nokia.com>

tests/manual/accessibility/hittest.qml

@@ -79,7 +79,6 @@ Rectangle {
     Rectangle {
         clip: chkClip.checked
         z: 2
-        Accessible.role: Accessible.Button
         id: rect1
         width: 100
         height: 100