[FSDP][BE] Clean up dead code from `clip_grad_norm_()` testing (#90250)
`FSDP.clip_grad_norm_()` is tested separately in `test_fsdp_clip_grad_norm.py`. This PR removes the dead non-run code from `common_fsdp.py` and `test_fsdp_core.py` related to `clip_grad_norm_()`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90250
Approved by: https://github.com/rohan-varma
