Revert "Add support for github.com/stuttter/ludicrousdb"

This reverts commit 7cf52a2a.

It turns out that the LudicrousDB documentation is slightly misleading,
and the 'dataset' callback actually receives exactly the same arguments as
with HyperDB, so there is no need for custom switching code.

db-backends.php

@@ -10,7 +10,7 @@ $r2db_hash_map = new Flexihash(null, R2DB_FLEXIHASH_REPLICAS);
  *
  * Implements a consistent hashing scheme using Flexihash.
  */
-function noblogs_hyperdb_callback($query, $wpdb) {
+function noblogs_db_callback($query, $wpdb) {
   global $r2db_hash_map;
   if (preg_match("/^{$wpdb->base_prefix}(\d+)_/", $wpdb->table, $matches)) {
     $blog_id = $matches[1];
@@ -18,22 +18,7 @@ function noblogs_hyperdb_callback($query, $wpdb) {
   }
 }
 
-/* Same as above but with a slightly different function
-   signature for https://github.com/stuttter/ludicrousdb */
-function noblogs_ludicrousdb_callback($table, $wpdb) {
-  global $r2db_hash_map;
-  if (preg_match("/^{$wpdb->base_prefix}(\d+)_/", $table, $matches)) {
-    $blog_id = $matches[1];
-    return $r2db_hash_map->lookup($blog_id);
-  }
-  return "global";
-}
-
-if (defined('R2DB_USE_LUDICROUSDB')) {
-  $wpdb->add_callback('noblogs_ludicrousdb_callback');
-} else {
-  $wpdb->add_callback('noblogs_hyperdb_callback');
-}
+$wpdb->add_callback('noblogs_db_callback');
 
 /* Load backend shards and add them to FlexiHash / $wpdb. */
 function noblogs_load_backends() {