chore: Remove unnecessary variables from env examples

Removed:
- REACT_APP_LOGGING_BROWSER_ENABLED (default is fine, naming is confusing)
- REACT_APP_LOGGING_LOG_LEVEL (defaults to "info")
- LOG_LEVEL (not used - code uses REACT_APP_LOGGING_LOG_LEVEL)
- ESLINT_USE_FLAT_CONFIG (build-time only, not needed for deployment)

Made DEBUG_* variables optional (commented out).