Bug 1084041 - Remove files after applying FOTA r=gsvelto

We should not have any leftover files after applying a FOTA package
update, so we should ensure we are removing them at the end of the
process:
 - MAR package that has been extracted to /data/local/...
 - update.zip that has been extracted to the sdcard for applying in
   recovery mode
nit: Do you really need this? You're already setting ``/sdcard`` as the default path in the ``add_argument()`` call below