unxfail test_keyboard as it has been passing for several builds

The bug that is referenced in the old xfail comment has not been fixed, but it doesn't seem to be manifesting in the test, and the test has been passing for several builds: http://qa-selenium.mv.mozilla.com:8080/view/B2G/job/b2g.unagi.gaia.nightly.ui.xfail/lastCompletedBuild/testReport/%28root%29/TestKeyboard/history/