nixos/test-driver: don't add QEMU_OPTS twice

As the TODO says, this is already included by the script.

If adding a device, including this again here would result in either
two devices being added, or, if they were explicitly named, an error
due to reuse of the name.

nixos/lib/test-driver/test_driver/machine.py

@@ -165,8 +165,6 @@ class StartCommand:
         )
         if not allow_reboot:
             qemu_opts += " -no-reboot"
-        # TODO: qemu script already catpures this env variable, legacy?
-        qemu_opts += " " + os.environ.get("QEMU_OPTS", "")
 
         return (
             f"{self._cmd}"