Get rid of stacked diversions in cluster-login-config

[jdreed]

Stacked diversions suck and are confusing. We can get rid of two of them by ripping out support for old-gdm (there are no longer any old-GDM releases on which anyone should be running cluster-login-config). The last one is the stacked diversion of /etc/pam.d/gdm. I wonder if we can get rid of this with clever use of pam conditionals and envrionment variables, and just ship the single diverted file in gdm-config, do something clever to add the pam_succeed_if line in the cluster world only. (And frankly, we don't need that in lightdm-config)