Disable python codegen for lists (#12300)

* Disable python codegen for lists

This code is not really used right now, doesn't work with nullable lists,
and is blocking new clusters from using nullable lists. Disable for now to unblock,
until it can be fixed or removed entirely.

* Regen code

Not strictly necessary (as only unused python code was removed for
just the case of lists) but presubmits require it.

* Restyle code

* Change comment and removal syntax