This removes all the code for add-on performance watching from the
perfmonitoring component. This should mean that for add-on
compartments, we no longer trigger jank or CPOW monitoring in the JS
engine. This should result in minor performance improvements. As a
result, about:performance no longer reports on add-on performance
(but still reports on web page performance).
It also removes the AddonWatchers.jsm module and the related Nightly-
only UI (disabled in the parent commit) and strings. This UI wasn't
ready for release, there wasn't sufficient data it was creating
value for users, and there was some evidence that it didn't always
correctly identify the cause of performance issues, thus potentially
leading to user confusion or annoyance. Removing it therefore seemed
the right thing to do.
MozReview-Commit-ID: LsRwuaUtq6L
