Remove classname Strings from XTS subsystem

THe XTS SubSystem has many classnames passed to Jandex as raw Strings. It would be better to use Class#getName as it is type-safe.

When this code was written, this was not possible, due to the nature of the order in which class-loading was done and the fact that the code in question was executed very early in the server boot. I don't remember the specific details.

However, something seems to have changed in WildFly, such that we can now use Class#getName on the classes we require. This issue it to replace all the classname Strings with Class#getName() and to also simplify the code under org.jboss.as.xts.jandex, as a lot of that verbose code was added to to cope with the limitation of not being able to use Class#getName().