**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
#
#
import copy
import json
import logging
import multiprocessing
import os
import pathlib
import re
import shlex
import subprocess
import sys
import warnings
from base64 import b64encode
from collections import OrderedDict
from configparser import _UNSET, ConfigParser, NoOptionError, NoSectionError  
from json.decoder import JSONDecodeError
from typing import Dict, List, Optional, Tuple, Union
import yaml
from cryptography.fernet import Fernet
from airflow.exceptions import AirflowConfigException
from airflow.secrets import DEFAULT_SECRETS_SEARCH_PATH, BaseSecretsBackend
from airflow.utils.module_loading import import_string
log = logging.getLogger(__name__)
if not sys.warnoptions:
    warnings.filterwarnings(action='default', category=DeprecationWarning, module='airflow')
    warnings.filterwarnings(action='default', category=PendingDeprecationWarning, module='airflow')
def expand_env_var(env_var):