Remove javax.* namespace code from testsuite/mixed-domain by removing the no-longer-relevant MixedDomainDeploymentTest.testJsfWorks()[1]. Change the installation / feature pack it tests to WildFly Preview pending subsequent conversion of std WildFly to javax.*.

[1] Not relevant because it tests that mixed-domain works with a release that didn't have the JSF subsystem, but we no longer support mixed-domain for such an old release.