nvfp4tensor: remove duplicated tests

Summary:

These tests were previously copied to `test_nvfp4_tensor.py`, but were
not deleted from `test_mx_tensor.py`. Deleting now to remove duplicate
code.

Test Plan:

```
pytest test/prototype/mx_formats/ -s -x
```

Reviewers:

Subscribers:

Tasks:

Tags:

ghstack-source-id: a91df7b
ghstack-comment-id: 3347278536
Pull Request resolved: #3090

Author: vkuzo