**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.
tests/core/test_config_templates.py
@parameterized.expand(
        [
            ("default_airflow.cfg",),
            ("default_test.cfg",),
        ]
    )
    def test_should_be_ascii_file(self, filename: str):
        with open(os.path.join(CONFIG_TEMPLATES_FOLDER, filename), "rb") as f:
            content = f.read().decode("ascii")
        self.assertTrue(content)
    @parameterized.expand(
        [
            (
                "default_airflow.cfg",
                DEFAULT_AIRFLOW_SECTIONS,
            ),
            (
                "default_test.cfg",
                DEFAULT_TEST_SECTIONS,
            ),
        ]
    )
    def test_should_be_ini_file(self, filename: str, expected_sections):
        filepath = os.path.join(CONFIG_TEMPLATES_FOLDER, filename)
        config = configparser.ConfigParser()
        config.read(filepath)
        self.assertEqual(expected_sections, config.sections())