Remove the deprecated hook_ranking() and the code invoking it

Problem/Motivation

There is a deprecated hook called hook_ranking() and line of code that invokes it in \Drupal\node\Plugin\Search\NodeSearch::getRankings(). We should clean it up.

Proposed resolution

Remove the hook and the code that invokes it in favor of the new hook.

Remaining tasks

1. In the core/modules/node/node.api.php file, remove the documentation block and the hook_ranking() function.
2. In \Drupal\node\Plugin\Search\NodeSearch::getRankings(), remove the line of code invoking the old hook.
3. In \Drupal\node\Plugin\Search\NodeSearch::getRankings(), change the implementation of invoking the new hook from an array addition to just a simple value assignment. From the "+=" assignment operation to just "=".

User interface changes

None.

API changes

hook_ranking() is deprecated in favor of hook_node_search_ranking() and needs to be removed.

Data model changes

None.