Remove unnecessary sector_size parameter from Get_New_Partition methods

The sector_size parameter is unnecessary as the value can be retrieved
from the sector size of the selected Partition object on which the
create new, copy & paste or resize/move operation is being performed.

For the create new and resize/move operations it is trivial as the
existing unallocated or in use Partition object on which the operation
is being perform already contains the correct sector size.  For the copy
& paste operation, which can copy across disk devices of different
sector sizes, we merely have to use the sector size of the existing
selected (destination) Partition object rather than copied (source)
Partition object.  Hence these relevant lines in the new code:

    Dialog_Partition_Copy::set_data(selected_partition, copied_partition)
        new_partition = copied_partition.clone();
        ...
        new_partition->sector_size = selected_partition.sector_size;