Unxfail test_cost_control_data_alert_mobile.py as it seems to be passing consistently again

The bug that I reopened [1] hasn't been commented on, but this test seems to now be passing consistently [2], and passes locally for me.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=864725
[2] http://qa-selenium.mv.mozilla.com:8080/view/B2G/job/b2g.unagi.gaia.nightly.ui.xfail/133/testReport/%28root%29/TestCostControlDataAlertMobile/history/?