GROOVY-6393: groovysh: load command without parsing xml

See https://jira.codehaus.org/browse/GROOVY-6393

Commands are still loaded if someone places a command.xml on the classpath. But that cml is removed from groovysh. And without it, command classes are instantiated in the code

Actually not sure how much time is saved, but the code is much simpler that way.