frontend: remove CACHE_DIR

Created by: keegancsmith

The code backing this variable has been unused for atleast a year. Removing this also allows us to simplify deployment, since right now we tell customers to assign ephemeral storage to frontend (but that will never be used!)

Test Plan: CI. I previously did a lot of validation that we can remove this (reading code, inspecting prod and dogfood environments for cache dir usage)

Part of https://github.com/sourcegraph/sourcegraph/issues/38934