**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
"""
        Validate that config values aren't invalid given other config values
        or system-level limitations and requirements.
        """
        is_executor_without_sqlite_support = self.get("core", "executor") not in (
            'DebugExecutor',
            'SequentialExecutor',
        )
        is_sqlite = "sqlite" in self.get('core', 'sql_alchemy_conn')
        if is_executor_without_sqlite_support and is_sqlite:
            raise AirflowConfigException(
                "error: cannot use sqlite with the {}".format(self.get('core', 'executor'))
            )
        if self.has_option('core', 'mp_start_method'):
            mp_start_method = self.get('core', 'mp_start_method')
            start_method_options = multiprocessing.get_all_start_methods()
            if mp_start_method not in start_method_options:
                raise AirflowConfigException(
                    "mp_start_method should not be "
                    + mp_start_method
                    + ". Possible values are "
                    + ", ".join(start_method_options)
                )
    def _using_old_value(self, old, current_value):