Stop copying displayed partition objects in activate_resize() (#750168)

The first actions of Win_GParted::activate_resize() were to create a
copy of the vector of partitions for the currently displayed device and
visually apply any pending operations.  Exactly this has already been
done in Win_GParted::Refresh_Visual() with the result now available in
the member variable display_partitions.  Stop this unnecessary partition
object copying and processing by just using display_partitions member
variable instead.

Bug 750168 - Reduce the amount of copying of partition objects