Revert "Made FlexAttention rewrite getitem calls to use aten.index in score_mod (#124799)"

This reverts commit acc4cbea395c25410c26d6fd3c88c072ce24c918.

Reverted https://github.com/pytorch/pytorch/pull/124799 on behalf of https://github.com/jeanschmidt due to checking if this diff introduced regressions on linux-focal-py3.11-clang10 and linux-focal-py3.8-clang10 ([comment](https://github.com/pytorch/pytorch/pull/124799#issuecomment-2076756876))

test/test_overrides.py

@@ -1387,28 +1387,6 @@ class TestTorchFunctionMode(TestCase):
 
         self.assertTrue(called)
 
-    def test_getitem_call(self):
-        # This failed because the parser thinks the function is called to()
-        # but it's actually called _parse_to()
-
-        called = False
-
-        class A(TorchFunctionMode):
-            def __torch_function__(self, func, types, args=(), kwargs=None):
-                nonlocal called
-                if kwargs is None:
-                    kwargs = {}
-                called = True
-                return func(*args, **kwargs)
-
-        a = torch.zeros(5)
-        b = torch.tensor(0)
-        with A():
-            a[b]
-
-        self.assertTrue(called)
-
-
     def test_distributions_bernoulli(self):
         # This failed because improper use of has_torch_function when
         # is_tensor_like should have been used instead, inside the