spec: janitorial: drop references to fedora 18

We no longer support anything below Fedora 19, so ancient complexities
can be ignored.

Change-Id: I268f5921942590bc661d1b29cd08d3e15a96826d
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33379
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>