the file is not required, gdm uses the standard GNOME theme if the selected theme is not present. On a standard Ubuntu installation the gdm Human theme is installed if you remove ubuntu-desktop you are responsive to making sure you keep packages you need installed
-- Session bug with gdm https://launchpad.net/bugs/67727 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
