Drop the emulated serial port from omarchy-iso-boot

QEMU's default COM1 is advertised to the guest via ACPI SPCR, the boot
stub appends console=uart,io,0x3f8 to the kernel cmdline, and Plymouth
deliberately forces its plain-text details mode whenever a serial console
is active. That made every omarchy-iso-boot test show an unthemed LUKS
prompt that real hardware (no serial port) never shows. Remove the serial
device so manual QEMU tests match real-hardware appearance.

omarchy-iso-test keeps its -serial file: logging on purpose — the harness
reads the console log and types blindly, so details mode is harmless there.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/omarchy-iso-boot

@@ -163,6 +163,7 @@ qemu-system-x86_64 \
   -device virtio-vga-gl \
   -display sdl,gl=on \
   -usb -device usb-tablet \
+  -serial none \
   "${NET_ARGS[@]}" \
   -boot menu=on \
   "${EXTRA_QEMU_ARGS[@]}"