Remove code generating comments in configuration file (#12941)

changelog.d/12941.misc

docs/.sample_config_header.yaml

docs/sample_config.yaml

synapse/config/_base.py

synapse/config/api.py

synapse/config/appservice.py

synapse/config/auth.py

synapse/config/background_updates.py

synapse/config/cache.py

synapse/config/captcha.py

synapse/config/cas.py

synapse/config/consent.py

synapse/config/database.py

synapse/config/emailconfig.py

synapse/config/federation.py

synapse/config/groups.py

synapse/config/jwt.py

synapse/config/key.py

synapse/config/logger.py

synapse/config/metrics.py