Configuration variables cleanup

[shaidar]

We are working on managing Micromasters Heroku configuration variables through Saltstack similar to how we manage those for the other apps. When looking through the different envs (CI, RC, and Production) and searching through settings.py, the keys below came into question. It would be great if someone can confirm whether those keys can be removed:

• [x] ELASTICSEARCH_DEFAULT_PAGE_SIZE
• [x] ELASTICSEARCH_HTTP_AUTH
• [x] ELASTICSEARCH_INDEX
• [x] ELASTICSEARCH_SHARD_COUNT
• [x] ELASTICSEARCH_URL
• [x] EXAMS_AUDIT_AWS_ACCESS_KEY_ID
• [x] EXAMS_AUDIT_AWS_SECRET_ACCESS_KEY
• [x] EXAMS_AUDIT_ENABLED
• [x] EXAMS_AUDIT_ENCRYPTION_FINGERPRINT
• [x] EXAMS_AUDIT_ENCRYPTION_PUBLIC_KEY
• [x] EXAMS_SSO_CLIENT_CODE
• [x] EXAMS_SSO_PASSPHRASE
• [x] EXAMS_SSO_URL
• [x] FEATURE_ENABLE_EDX_EXAMS
• [ ] FEATURE_ENABLE_PROGRAM_LETTER
• [ ] FEATURE_MITXONLINE_LOGIN
• [x] FEATURE_PEARSON_EXAMS_SYNC
• [x] FEATURE_TURN_PAYMENT_OFF
• [x] FEATURE_USE_COMBINED_FINAL_GRADE
• [ ] MICROMASTERS_ECOMMERCE_EMAIL