Clean up stale TORCH_NCCL_AVOID_RECORD_STREAMS and avoidRecordStreams_ (#194701)

TORCH_NCCL_AVOID_RECORD_STREAMS has been deprecated and ignored for more than a year - stashing via stashed_for_allocator_safety_ is now unconditional. The member avoidRecordStreams_ in ProcessGroupNCCL was likewise unused, but 13 comment sites and the docs still referenced both the env var and the flag.

No functional change - the stashing path was already always active.

Test Plan:
```
grep -rn "TORCH_NCCL_AVOID_RECORD_STREAMS\|avoidRecordStreams_" --include="*.cpp" --include="*.hpp" --include="*.md"
# -> 0 hits

grep -n "stashing note" torch/csrc/distributed/c10d/ProcessGroupNCCL.cpp
# -> 12 sites

lintrunner -a
# -> ok No lint issues.
```

Co-authored with Muse Code powered by Meta Muse Spark.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/194701
Approved by: https://github.com/kapilsh, https://github.com/fduwjj