**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
"""Historical as_dict"""
    warnings.warn(
        "Accessing configuration method 'as_dict' directly from the configuration module is "
        "deprecated. Please access the configuration from the 'configuration.conf' object via "
        "'conf.as_dict'",
        DeprecationWarning,
        stacklevel=2,
    )
    return conf.as_dict(*args, **kwargs)
def set(*args, **kwargs):  
    """Historical set"""
    warnings.warn(
        "Accessing configuration method 'set' directly from the configuration module is "
        "deprecated. Please access the configuration from the 'configuration.conf' object via "
        "'conf.set'",
        DeprecationWarning,
        stacklevel=2,
    )
    return conf.set(*args, **kwargs)
def ensure_secrets_loaded() -> List[BaseSecretsBackend]:
    """
    Ensure that all secrets backends are loaded.
    If the secrets_backend_list contains only 2 default backends, reload it.
    """
    
    if len(secrets_backend_list) == 2:
        return initialize_secrets_backends()
    return secrets_backend_list
def get_custom_secret_backend() -> Optional[BaseSecretsBackend]: