**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
"""
        Remove an option if it exists in config from a file or
        default config. If both of config have the same option, this removes
        the option in both configs unless remove_default=False.
        """
        if super().has_option(section, option):
            super().remove_option(section, option)
        if self.airflow_defaults.has_option(section, option) and remove_default:
            self.airflow_defaults.remove_option(section, option)
    
    def getsection(self, section: str) -> Optional[Dict[str, Union[str, int, float, bool]]]:
        """
        Returns the section as a dict. Values are converted to int, float, bool
        as required.
        :param section: section from the config
        :rtype: dict
        """
        
        if section not in self._sections and section not in self.airflow_defaults._sections:  
            return None
        
        _section = copy.deepcopy(self.airflow_defaults._sections[section])  
        if section in self._sections:  
            _section.update(copy.deepcopy(self._sections[section]))  
        section_prefix = f'AIRFLOW__{section.upper()}__'
        for env_var in sorted(os.environ.keys()):
            if env_var.startswith(section_prefix):