**Apache Airflow version**: 2.0.0b* / master


**What happened**:

`[ldap]` section in `airflow.cfg` is not applicable anymore in 2.0 and `master`, because the LDAP authentication (for webserver and API) is handled by FAB, and the configuration for this is handled by `webserver_config.py` file.

![airflow_–_default_airflow_cfg](https://user-images.githubusercontent.com/11539188/99289209-ef210700-283c-11eb-948e-16811ba09293.png)


**What you expected to happen**:

The `[ldap]` section should be removed from `airflow/config_templates/default_airflow.cfg` and `airflow/config_templates/config.yml` (and some other applicable files).

Otherwise leaving this section there will be a big confusion for users.
airflow/configuration.py
"""Get Config option values from Secret Backend"""
        fallback_key = key + '_secret'
        
        if (section, key) in self.sensitive_config_values:
            if super().has_option(section, fallback_key):
                secrets_path = super().get(section, fallback_key)
                return _get_config_value_from_secret_backend(secrets_path)
        return None
    def get(self, section, key, **kwargs):
        section = str(section).lower()
        key = str(key).lower()
        deprecated_section, deprecated_key = self.deprecated_options.get((section, key), (None, None))
        option = self._get_environment_variables(deprecated_key, deprecated_section, key, section)
        if option is not None:
            return option
        option = self._get_option_from_config_file(deprecated_key, deprecated_section, key, kwargs, section)
        if option is not None:
            return option
        option = self._get_option_from_commands(deprecated_key, deprecated_section, key, section)
        if option is not None:
            return option
        option = self._get_option_from_secrets(deprecated_key, deprecated_section, key, section)
        if option is not None:
            return option
        return self._get_option_from_default_config(section, key, **kwargs)
    def _get_option_from_default_config(self, section, key, **kwargs):