Remove dummy StyleItem inside Slider

We should not bind the size to this dummy item, since the delegate
can be changed leading to a wrong implicit size in that case.

Change-Id: Ibeff637dba1ea485abdb6801685f5262a3843ca7
Reviewed-by: Daker Fernandes Pinheiro <daker.pinheiro@openbossa.org>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@nokia.com>