Remove unneeded flex_decoding.patch (#4962)

```
-            max(
-                next_power_of_2(
-                    V.graph.sizevars.size_hint(
-                        seq_len_q,
-                        fallback=torch._inductor.config.unbacked_symint_fallback,  # type: ignore[arg-type]
-                    )
-                    * gqa_shared_heads
-                ),
-                1 if torch.xpu.is_available() else 16,
+            next_power_of_2(
+                V.graph.sizevars.size_hint(
+                    seq_len_q,
+                    fallback=torch._inductor.config.unbacked_symint_fallback,  # type: ignore[arg-type]
+                )
+                * gqa_shared_heads
```
On xpu, the results are equivalent, `max(next_power_of_2, 1) ==
next_power_of_2`.

CI:
https://github.com/intel/intel-xpu-backend-for-triton/actions/runs/17266774537/job/49000662056
(no regression)

Signed-off-by: Whitney Tsang <[email protected]>

Author: whitneywhtsang

scripts/patch-pytorch.sh

@@ -36,4 +36,3 @@ echo "Applying PyTorch patches in $REPO_ROOT"
 
 # put your patch applies here
 apply_patch ./patch/flex_attn_143553.patch
-apply_patch ./patch/flex_decoding.patch