Remove xfail from test_user_can_go_back_from_settings_page().

The test was xfailed for the reason mentioned in the following comment [1]. I have checked the latest builds and in most of them the test XPasses. In the situations where it fails it is for other reasons than the one mentioned in #381.
I removed the xfail so that the job fails if the test encounters other errors.

[1] https://github.com/mozilla/marketplace-tests/issues/381#issuecomment-27481352