Oz: don't hardcode the image size unit as 'G'

This has never been necessary, because oz has always treated a
size with no unit as being in gibibytes. After
https://github.com/clalancette/oz/pull/310 it will be actively
harmful, because it will prevent us from using that new ability
of oz to specify a size using any other unit, as we have a
specific reason to want:
https://bugzilla.redhat.com/show_bug.cgi?id=2247611

Signed-off-by: Adam Williamson <awilliam@redhat.com>

builder/kojid

@@ -4101,7 +4101,7 @@ class OzImageTask(BaseTaskHandler):
         template += """</os>
     <description>%s OS</description>
     <disk>
-        <size>%sG</size>
+        <size>%s</size>
     </disk>
 </template>
 """ % (name, self.opts.get('disk_size'))  # noqa: E501