Remove `tree_map` deprecation filter after Flax upgrades minimum Python version to 3.10

[Marcus Chiam]

Context:

• As of JAX 0.4.26, jax.tree_map is deprecated
• #3823 renames all jax.tree_map usages to jax.tree_util.tree_map in Flax, however we get an error in CI because of a CLU dependency
• After fixing CLU and pushing a new release, the error remains on CI for Python 3.9 tests
  • this is because Flax enforces an earlier version of CLU (before the tree_map fix) on python versions less than 3.10, since the match-case syntax used by CLU is only available in Python 3.10 or greater
  • i.e. because Flax supports a minimum Python version of 3.9, Flax must use an earlier CLU version (where the CLU fix has not landed yet), since the current CLU version with the fix uses a python syntax that isn't available until Python 3.10
• Our current solution is to add a deprecation warning filter in #3828

Once Flax upgrades its minimum Python version to 3.10, we should remove the deprecation warning filter and remove enforcing an earlier version of CLU.