nixos/virtualisation: remove hard-coded virtio-gpu-pci device from aarch machines

In 9e78baf5c7, Aarch machines were added
the `-device virtio-gpu-pci` argument, to mirror `-vga std` option that
was added unconditionally to virtualised x86_64 machines. Since then,
in a7ca287ecb, the `-vga std` option was
removed, because that is the default value of this option embedded in
QEMU since version 2.2.

Removing it from the hard-coded list of qemu options makes it possible
for the user to choose their own `-vga` value. Similarly, for Aarch
virtualised machines, choosing unconditionally the `-device
virtio-gpu-pci` option instead of e.g `-device virtio-gpu-gl-pci`,
can be bothersome too - see:

https://discourse.nixos.org/t/test-an-aarch64-linux-vm-wayland-compositor-from-an-x86-64-linux-host/77416/2

nixos/modules/virtualisation/qemu-vm.nix

@@ -1380,7 +1380,6 @@ in
         "-device usb-tablet,bus=usb-bus.0"
       ])
       (mkIf pkgs.stdenv.hostPlatform.isAarch [
-        "-device virtio-gpu-pci"
         "-device usb-ehci,id=usb0"
         "-device usb-kbd"
         "-device usb-tablet"