Make the .who command more readable

Make the .who command (`l1j.server.server.command.executor.L1Who`) more readable by replacing the magic numbers in the `#getSex` and `#getClass` methods with the corresponding `l1j.server.server.model.Instance.L1PcInstance.CLASSID_*` constants, e.g. `L1PcInstance.CLASSID_PRINCE`.

Also, remove the unused `l1j.server.server.model.classes.L1ClassId` class.