**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
if super().has_option(section, key):
            
            
            return expand_env_var(super().get(section, key, **kwargs))
        if deprecated_section:
            if super().has_option(deprecated_section, deprecated_key):
                self._warn_deprecate(section, key, deprecated_section, deprecated_key)
                return expand_env_var(super().get(deprecated_section, deprecated_key, **kwargs))
        return None
    def _get_environment_variables(self, deprecated_key, deprecated_section, key, section):
        
        option = self._get_env_var_option(section, key)
        if option is not None:
            return option
        if deprecated_section:
            option = self._get_env_var_option(deprecated_section, deprecated_key)
            if option is not None:
                self._warn_deprecate(section, key, deprecated_section, deprecated_key)
                return option
        return None
    def getboolean(self, section, key, **kwargs):