fix: remove duplicate kv_indptr loop in benchmark

[yurekami]

Summary

• Removes duplicate loop that incorrectly extended kv_indptr a second time in run_flash_infer() function
• The loop at lines 100-101 was iterating over cache_seqlens[1:] and appending to kv_indptr after the main loop (lines 94-99) had already built the complete array
• This caused kv_indptr to have 2*b elements instead of the correct b+1 elements

Test plan

• Code review - the duplicate loop was clearly redundant
• The main loop already handles all batch elements correctly

🤖 Generated with Claude Code