[FEATURE] Remove Update Batch Size

[RuanJohn]

Please describe the purpose of the feature. Is it related to a problem?

This is somewhere between a feature and maintenance. Having to vmap an extra time is not necessarily as good an idea as we thought when we started rewriting Mava in JAX. I think it would be better to rather be able to use twice as many environments and then only have an outer pmap. It'll also remove a non-standard config option.

Describe the solution you'd like

Remove the update_batch_size config option and update all systems accordingly.

How do we know when implementation of this feature is complete?

Checklist:

• All systems are benchmarked and can get similar performance.