Config split patches for views do not remove handlers

Problem/Motivation

When there is a view that is partially splitted, and in the partial split a views handler needs to be removed, the handler does not get removed and appears as broken.

This happens in a multi-site where one handler is removed for specific site requirements.

Steps to reproduce

- Create a view in default configuration
- Create a config split that marks this view as partial split.
- Remove a handler and export this change to the config split.

Now, import this new configuration in one of these:
- Other environment that does not have the new configuration
- The same environment but with the a database loaded that does not have this new configuration.

Proposed resolution

Implement hook_config_schema_info_alter so that the handler mapping schema has the 'plugin_id' as the patch index. Example with view sorts:

/**
 * Implements hook_config_schema_info_alter().
 */
function mysite_config_config_schema_info_alter(&$definitions) {
  if (isset($definitions['views_sort'])) {
    $definitions['views_sort']['settings']['patch index'] = 'plugin_id';
  }
}

With this, throw a exception into the mergeArray method at ConfigPatchMerge and capture it with a try/catch so that if a mapping configuration does not have its patch index, it gets removed from $result.

Remaining tasks

- Tests.
- Check if the config split generated patch contents are still correct, as the current solution only acts in the configuration import process and not in the configuration export.