remove flax from `documentation_tests.txt`
# What does this PR do?

#21009 added `src/transformers/generation/flax_utils.py` to `documentation_tests.txt`, but CI image doesn't have `jax/flax` installed. The whole doctest suite failed, and reported 0 failure.

This PR removes this file from `documentation_tests.txt`. The CI image used is the same as the scheduled CI, and it's intended not to have `jax/flax` in this image.

We can decide if to use a separate image though.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Ah, this explains why the previous example had several issues 😅 

@ydshieh We don't plan to add FLAX to the doctests, correct?<|||||>@gante Not a no from me. I think it's good to make sure the examples work (this is in the range of maintenance mode 😄  ). But I would like to have a yes from @sgugger and @LysandreJik before working on it.<|||||>Not worth any work right now given the usage IMO. There is plenty to do with more priority :-) 