Cleanup binders in GCE: don't use sneaky injection. don't redundantly check null

Private injection requires reflection and limits DI choices. Binders don't need to check null as jclouds already checks null for any param not marked nullable.
less lines, explicitly states that this is an injected constructor. This is
a style I've seen common in recent google and square projects, since they
invented Inject, and the former style looks uglier, I prefer to follow.