Admin: Don't call dead Mode.__call__ code

In 9eb3db84, Bcfg2.Server.Admin.Mode.__call__() was changed from
pass to raise a NotImplementedError.  This causes bcfg2-admin
compare and bcfg2-admin snapshots to fail because they call
Bcfg2.Server.Admin.Mode.__call__() right away.  Since that method
didn't do anything anyway, it seems expediant to just avoid the call
altogether.
Merge pull request #144 from fennm/admin-avoid-NotImplementedError

Admin: Don't call dead Mode.__call__ code