A bunch of our tests "return false" from the handler, but it's unnecessary since our events aren't bubbling (since bug 608630) or cancellable (since bug 770945).
Also removes a couple of tests for completed variables that are declared false and never set to true.
Deletes 70 lines. Green locally, pushed to try: https://tbpl.mozilla.org/?tree=Try&rev=9b44c69194fc
Created attachment 651995 [details] [diff] [review]