docs: drop redundant intro lines

docs/design/skip_softmax_sage_trtllm_gen.md

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 # SAGE + Skip-Softmax — API Design & Test Plan
 
-Backend: **trtllm-gen** (FlashInfer). Primary metric: **LPIPS ↓** vs BF16 dense. Speedup vs BF16 dense.
-Order: **(1) SAGE → (2) Skip → (3) SAGE + Skip → (4) cross-backend.**
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 # Part 1 — API Design
 
 *How the user uses the trtllm-gen backend.*