Bug 1025240 - test-native-options should cleanup the preference branches that it creates

it's a bit silly to delete the prefs that we just asserted were not defined.. ;)

anyway, this test doesn't test things anymore. 

`simple.on` would emit events for the global `self.preferencesBranch` (of our test addon), not for this local one "no-prefs@jetpack".

my self-imposed limitation while porting these unit-tests from python was not to change `package.json` files like you did above. so this test used the global preferences branch, which is why it used events, since i couldn't simply iterate over `simple` prefs to check that they are empty.

i don't think any change to this test is even required, as it's sure not to leave prefs behind. if you still want to delete the branch, you can switch to using the PrefService branch directly to assert it's empty, without `simple-prefs` and events.