revert unused change

python/paddle/fluid/optimizer.py

@@ -292,8 +292,6 @@ class Optimizer(object):
         This method combines interface `append_backward()` and
         `create_optimization_pass()` into one.
         """
-        with program_guard(loss.block.program, startup_program):
-
         params_grads = append_backward(loss, parameter_list, no_grad_set,
                                        [error_clip_callback])