Env: remove List.*assq functions

As a general rule, we don't want to rely on physical equality in the protocol. (E.g., == is not in the environment.)

This MR removes List.*assq functions which are just List.*ass ~eq:( == ).

NOTE: this MR modifies env-v9 which is being released as part of octez-v17. Maybe we will need to change this MR to target env-v10 instead.