**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 old.search(current_value) is not None
    def _update_env_var(self, section, name, new_value):
        
        
        env_var = self._env_var_name(section, name)
        os.environ.pop(env_var, None)
        self.set(section, name, new_value)
    @staticmethod
    def _create_future_warning(name, section, current_value, new_value, version):
        warnings.warn(
            'The {name} setting in [{section}] has the old default value '
            'of {current_value!r}. This value has been changed to {new_value!r} in the '
            'running config, but please update your config before Apache '
            'Airflow {version}.'.format(
                name=name, section=section, current_value=current_value, new_value=new_value, version=version
            ),
            FutureWarning,
        )
    @staticmethod
    def _env_var_name(section, key):