remove xfail for the HTTP 200 tests

The HTTP 200 redirect tests appear to be passing again on:
* mozillians.org (prod)
* mozillians.allizom.org (stage)
* mozillians-dev.allizom.org (dev)

        mbrandt$ py.test --baseurl=https://mozillians-dev.allizom.org tests/test_redirects.py -k 200
        ==== 1 tests deselected by "-k200 -m 'nondestructive'" ====
        ==== 1 passed, 1 deselected in 0.64 seconds ====