Description

This is a pure refactoring of lms/envs/production.py, removing several redundant statements that have accrued over the years as the platform moved from python-only, to python+json, to python+json+yaml, to today's python+yaml setup.

Notes on some of the more involved refactorings:

• AWS Locals Load block is handled by the YAML loading at the very top, we don't need to re-load it since there were no changes to those settings between the YAML loading at the top and this section.
• MKTG_URL_OVERRIDES, we drop doing any overrides and remove it from the merge list beacuse the default value in
  lms/envs/common.py is empty. So the update is a no-op and is the same as just loading this data directly from the YAML config.
• CODE_JAIL block, we've been overriding the entire dict if it is in your YAML config, so then going through and updating
  the individual values is not necessary.
• SSL_AUTH_EMAIL_DOMAIN and SSL_AUTH_DN_FORMAT_STRING are not used anywhere in the openedx org, looks like they were used by the old dashboard djangoapp and can probably be deleted but might be used by plugins so not removing for now to keep the change backward compatible.
• DEFAULT_FILE_STORAGE, previously two of the braches were no-ops so we only keep the one branch we need for when we want to update DEFAULT_FILE_STORAGE automatically if AWS keys are set.

Testing Instructions

We confirmed that the django settings exposed by lms/envs/production.py were identical using: https://gist.github.com/kdmccormick/9dd58fca319a6372776cd47d9136ffd3

git checkout kdmccormick/lms-envs-production-cleanup
git restore upstream/master -s lms/envs/production.py
mv lms/envs/production.py lms/envs/production_old.py
git restore lms/envs/production.py
./diff_settings.sh lms/envs/production_old.py lms/envs/production.py  # should yield a small diff of just random values