core: Remove redundant JOIN statement in a storage procedure

Removed JOIN in fn_get_actual_images_size_by_storage, the join with between
images_storage_domain_view and disk_image_dynamic since the field
actual_size used in the statement already exists in
images_storage_domain_view

Change-Id: I4f7096ef43876f3b2b96f61a60620e0e33a4e6db
Signed-off-by: Tal Nisan <tnisan@redhat.com>