nixos/tests/test-driver: cleanup "dead" code (USE_SERIAL)

At nixpkgs root:

`rg redirectSerial ./` does not result in any other match
nor does
`rg USE_SERIAL ./` except for an unrelated match in:

pkgs/tools/graphics/argyllcms/default.nix

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

@@ -907,7 +907,6 @@ class Machine:
 def create_machine(args: Dict[str, Any]) -> Machine:
     global log
     args["log"] = log
-    args["redirectSerial"] = os.environ.get("USE_SERIAL", "0") == "1"
     return Machine(args)