[FSDP] Remove `utils.py` (moved to `_utils.py`) (#86528)
I messed up my git with an earlier PR, where I did not actually remove `utils.py` when moving it to `_utils.py`. This removes `utils.py`, which is now outdated and unused.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86528
Approved by: https://github.com/H-Huang
