Disable cell data in cleanup

I found a telephony test that has EDGE active:
http://qa-selenium.mv.mozilla.com:8080/job/b2g.unagi.gaia.v1-train.ui/372/testReport/junit/%28root%29/TestDialer/attachments/test_dialer_make_call_screenshot.png

Because of that I added that in cleanup to make sure we run on a clean environment