It's no longer needed, now that legacy extensions aren't supported.
Pieces removed include the following.
- The "load-extension-default" observer notification.
- The code for reading defaults/preferences/*.js from extensions.
- The unit test for this stuff.
- A crash reporter annotation relating to very long prefs set by add-ons.
- All references to "ExtPrefDL".
MozReview-Commit-ID: KMBoYn3uZ3x
--HG--
extra : rebase_source : 4dc8ffd425c6cdf06806409090c4f9d04a64930b