class PostfixadminPrune
This class does absolutely nothing except allow us to blindly instantiate classes based on the mode name and configured plugins.
We don't need the ability to remove “left over” Postfixadmin users or domains, since “left over” is with respect to what's in Postfixadmin itself. The other pruning plugins check themselves against Postfixadmin – it doesn't make sense to check Postfixadmin against itself.