removed obsolete field "width" from VNativeButton. (#12825)

The field width of VNativeButton was written when setWidth() was invoked,
but it was never read anywhere. Removing it saves cpu time and memory.
This field was not marked as internal and it was protected, if anybody needs
it, they can easily add it to their classes.

Change-Id: I9f2bb9b5426b25ba36b3414cc70d2298e515ac23