Delete flax.optim.rst

[ppwwyyxx]

This module no longer exists

What does this PR do?

Fixes # (issue)

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).
• This change is discussed in a Github issue/
  discussion (please add a
  link).
• The documentation and docstrings adhere to the
  documentation guidelines.
• This change includes necessary high-coverage tests.
  (No quality testing = no merge!)

[cgarciae]

@andsteing can we delete this now?

[8bitmp3]

cc @jheek @levskaya @IvyZX

[levskaya]

Thanks for catching this!

[levskaya]

@cgarciae - I see some doctest failures of the form AttributeError: module 'jax.test_util' has no attribute 'check_eq' <-- is this a new issue cropping up? (obviously unrelated to this PR)

[cgarciae]

@levskaya never seen this. Is it happening on main too?