nixos/nvidia: remove obsolete GDM on Wayland assertion
nixos/nvidia: remove obsolete GDM on Wayland assertion

GDM enables Wayland on supported platforms automatically (see ${gnome.gdm}/lib/udev/rules.d/61-gdm.rules), so we removed the `gdm.nvidiaWayland` option.

You will still need `hardware.nvidia.modesetting.enable = true;` with `nvidia` driver, though.

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