**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
:type include_env: bool
        :param include_cmds: Should the result of calling any *_cmd config be
            set (True, default), or should the _cmd options be left as the
            command to run (False)
        :type include_cmds: bool
        :param include_secret: Should the result of calling any *_secret config be
            set (True, default), or should the _secret options be left as the
            path to get the secret from (False)
        :type include_secret: bool
        :rtype: Dict[str, Dict[str, str]]
        :return: Dictionary, where the key is the name of the section and the content is
            the dictionary with the name of the parameter and its value.
        """
        config_sources: Dict[str, Dict[str, str]] = {}
        configs = [
            ('default', self.airflow_defaults),
            ('airflow.cfg', self),
        ]
        self._replace_config_with_display_sources(config_sources, configs, display_source, raw)
        
        if include_env:
            self._include_envs(config_sources, display_sensitive, display_source, raw)
        
        if include_cmds:
            self._include_commands(config_sources, display_sensitive, display_source, raw)
        
        if include_secret:
            self._include_secrets(config_sources, display_sensitive, display_source, raw)
        return config_sources
    def _include_secrets(self, config_sources, display_sensitive, display_source, raw):