Remove image size fixing for Fedora cloud image

Since patch set 'Increase the size heuristic for images' to diskimage-builder
(https://review.openstack.org/#/c/53818/) was merged we don't need anymore
to set variable 'DIB_IMAGE_SIZE' for Fedora cloud image.
This variable is needed for CentOS cloud image only.

Change-Id: Ic02a50eb36d0213a207549a01b47e72418a19396