Remove Migrate Drupal settings from settings.php

Problem/Motivation

default.settings.php still has some Migrate Drupal related settings:

/**
 * Node migration type.
 *
 * This is used to force the migration system to use the classic node migrations
 * instead of the default complete node migrations. The migration system will
 * use the classic node migration only if there are existing migrate_map tables
 * for the classic node migrations and they contain data. These tables may not
 * exist if you are developing custom migrations and do not want to use the
 * complete node migrations. Set this to TRUE to force the use of the classic
 * node migrations.
 */
$settings['migrate_node_migrate_type_classic'] = FALSE;

/**
 * The default settings for migration sources.
 *
 * These settings are used as the default settings on the Credential form at
 * /upgrade/credentials.
 *
 * - migrate_source_version - The version of the source database. This can be
 *   '6' or '7'. Defaults to '7'.
 * - migrate_source_connection - The key in the $databases array for the source
 *   site.
 * - migrate_file_public_path - The location of the source Drupal 6 or Drupal 7
 *   public files. This can be a local file directory containing the source
 *   Drupal 6 or Drupal 7 site (e.g /var/www/docroot), or the site address
 *   (e.g http://example.com).
 * - migrate_file_private_path - The location of the source Drupal 7 private
 *   files. This can be a local file directory containing the source Drupal 7
 *   site (e.g /var/www/docroot), or empty to use the same value as Public
 *   files directory.
 *
 * Sample configuration for a drupal 6 source site with the source files in a
 * local directory.
 *
 * @code
 * $settings['migrate_source_version'] = '6';
 * $settings['migrate_source_connection'] = 'migrate';
 * $settings['migrate_file_public_path'] = '/var/www/drupal6';
 * @endcode
 *
 * Sample configuration for a drupal 7 source site with public source files on
 * the source site and the private files in a local directory.
 *
 * @code
 * $settings['migrate_source_version'] = '7';
 * $settings['migrate_source_connection'] = 'migrate';
 * $settings['migrate_file_public_path'] = 'https://drupal7.com';
 * $settings['migrate_file_private_path'] = '/var/www/drupal7';
 * @endcode
 */
# $settings['migrate_source_connection'] = '';
# $settings['migrate_source_version'] = '';
# $settings['migrate_file_public_path'] = '';
# $settings['migrate_file_private_path'] = '';

Steps to reproduce

Proposed resolution

We should check that there are no remaining uses of these in core following #3572280: Remove Migrate Drupal, and remove them if we can.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet