Clean out compiled protos.

def Clean():
  """Clean out compiled protos."""
  # Find all the compiled proto files and unlink them.
  for (root, _, files) in os.walk(ROOT):
    for filename in files:
      full_filename = os.path.join(root, filename)
      if full_filename.endswith("_pb2.py") or full_filename.endswith(
          "_pb2.pyc"):
        os.unlink(full_filename)