Remove test failure expectation for Firefox 31

Firefox bug https://bugzilla.mozilla.org/show_bug.cgi?id=1024926 has now been fixed and pushed to Beta.
Failure expectations can now be removed.
Firefox Aurora is not keeping track with Firefox Beta version + 1 and still contains the bug. Disabled Firefox Aurora channel from turning the tree red and sent an email to the maintainers of the firefox-aurora repository: https://launchpad.net/~ubuntu-mozilla-daily/+archive/ubuntu/firefox-aurora