Fold exim::rt into role::rt

exim::rt has currently a single caller, from the role::rt role class.
Moreover, it doesn't make sense to have this logic embedded into the
mail manifest or as an exim subclass and it makes more sense to fold
into the role class, similar to how role::otrs is set up.

Change-Id: I3acb44fc8add7a8d0b083521be04546c96ffb317