Fix grubby traceback on fresh F17 system (#872088)

bootloader.default_entry() causes a traceback if your grub2.cfg has:

  set default="${saved_entry}"

which is the F17 default, apparently. But we don't need to check the
default entry for anything except debugging, so.. just drop those lines.

fedup/download.py

@@ -297,8 +297,6 @@ def setup_upgraderoot():
 def modify_bootloader(kernel, initrd):
     log.info("reading bootloader config")
     bootloader = Grubby()
-    default = bootloader.default_entry()
-    log.info("default boot entry: \"%s\", %s", default.title, default.root)
 
     # avoid duplicate boot entries
     for e in bootloader.get_entries():