**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
return optionstr
    def __init__(self, default_config=None, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.airflow_defaults = ConfigParser(*args, **kwargs)
        if default_config is not None:
            self.airflow_defaults.read_string(default_config)
        self.is_validated = False
    def _validate(self):
        self._validate_config_dependencies()
        for section, replacement in self.deprecated_values.items():
            for name, info in replacement.items():
                old, new, version = info
                current_value = self.get(section, name, fallback=None)
                if self._using_old_value(old, current_value):
                    new_value = re.sub(old, new, current_value)
                    self._update_env_var(section=section, name=name, new_value=new_value)
                    self._create_future_warning(
                        name=name,
                        section=section,
                        current_value=current_value,
                        new_value=new_value,
                        version=version,
                    )
        self.is_validated = True
    def _validate_config_dependencies(self):