Remove Flax OPT from `documentation_tests.txt`
# What does this PR do?

Remove Flax OPT from `documentation_tests.txt` for now. The test uses the docker image `huggingface/transformers-all-latest-gpu` which has no JAX/Flax installed.


[Failed job run](https://github.com/huggingface/transformers/runs/7551001967?check_suite_focus=true)
Error:

```python
src/transformers/models/opt/modeling_flax_opt.py:20: in <module>
    import flax.linen as nn
E   ModuleNotFoundError: No module named 'flax'
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>@ArthurZucker This file is used to test the docstrings in model files and some documentation files. It is something different from the usual model testing.