**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
('metrics', 'statsd_host'): ('scheduler', 'statsd_host'),
        ('metrics', 'statsd_port'): ('scheduler', 'statsd_port'),
        ('metrics', 'statsd_prefix'): ('scheduler', 'statsd_prefix'),
        ('metrics', 'statsd_allow_list'): ('scheduler', 'statsd_allow_list'),
        ('metrics', 'stat_name_handler'): ('scheduler', 'stat_name_handler'),
        ('metrics', 'statsd_datadog_enabled'): ('scheduler', 'statsd_datadog_enabled'),
        ('metrics', 'statsd_datadog_tags'): ('scheduler', 'statsd_datadog_tags'),
        ('metrics', 'statsd_custom_client_path'): ('scheduler', 'statsd_custom_client_path'),
    }
    
    
    deprecated_values = {
        'core': {
            'hostname_callable': (re.compile(r':'), r'.', '2.1'),
        },
        'webserver': {
            'navbar_color': (re.compile(r'\A#007A87\Z', re.IGNORECASE), '#fff', '2.1'),
        },
        'email': {
            'email_backend': (
                re.compile(r'^airflow\.contrib\.utils\.sendgrid\.send_email$'),
                r'airflow.providers.sendgrid.utils.emailer.send_email',
                '2.1',
            ),
        },
    }
    
    
    
    def optionxform(self, optionstr: str) -> str: