[test]: drop redundant attn_qat_infer capability test (superseded by hao-ai-lab#1647)

The hardening commit added fastvideo/tests/attention/test_attn_qat_infer_
capability.py to assert is_attn_qat_infer_available() gates on device
capability. hao-ai-lab#1647 (same author) since landed test_attn_qat_infer_arch_gate.py
on main — a strict superset that parametrizes the full sm_12x-CUTLASS /
sm_100-103-FA4 resolution (incl. the same is_attn_qat_infer_available assertion)
plus receipts, forward-dispatch safety, memoization and opcheck. The older test
also asserted the now-stale (10,0)->False (pre-hao-ai-lab#1647 FA4 support), reddening CI.
Remove it and its pr_test lane entry; arch_gate.py is the single source of truth.

Author: Mister-Raggs