**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 f'AIRFLOW__{section.upper()}__{key.upper()}'
    def _get_env_var_option(self, section, key):
        
        env_var = self._env_var_name(section, key)
        if env_var in os.environ:
            return expand_env_var(os.environ[env_var])
        
        env_var_cmd = env_var + '_CMD'
        if env_var_cmd in os.environ:
            
            if (section, key) in self.sensitive_config_values:
                return run_command(os.environ[env_var_cmd])
        
        env_var_secret_path = env_var + '_SECRET'
        if env_var_secret_path in os.environ:
            
            if (section, key) in self.sensitive_config_values:
                return _get_config_value_from_secret_backend(os.environ[env_var_secret_path])
        return None
    def _get_cmd_option(self, section, key):
        fallback_key = key + '_cmd'
        
        if (section, key) in self.sensitive_config_values:
            if super().has_option(section, fallback_key):
                command = super().get(section, fallback_key)
                return run_command(command)
        return None
    def _get_secret_option(self, section, key):