Clean up debathena-xsession and kill off "login without customizations"

[jdreed]

debathena-xsession is kind of a mess. As part of killing off "Login Without Customizations" (in favor of the new account repair wizard), we should also clean it up a bit. In particular, we don't need some of the gdm-specific hacks that are in there. We should also see if we can combine a number of the xsession.d scripts, and possibly remove the separate .sh scripts and just put better conditionals in Xsession.d so that it's a no-op if the package isn't installed.