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
Yes, I've seen some devices where the device tree configuration may be wrong, so this is not 100% required, but it makes sure we won't generate improper paths.