MDL-68726 Search: Stop Solr 'optimize' behaviour

The optimize feature in Solr is usually considered harmful, especially
prior to Solr 7.5.

This change simply removes the optimize implementation from the Solr
engine.

search/classes/engine.php

@@ -394,6 +394,8 @@ public function area_index_complete($searcharea, $numdocs = 0, $fullindex = fals
      */
     public function optimize() {
         // Nothing by default.
+        mtrace('The ' . get_string('pluginname', $this->get_plugin_name()) .
+                ' search engine does not require automatic optimization.');
     }
 
     /**

search/engine/solr/classes/engine.php

@@ -1203,15 +1203,6 @@ public function file_indexing_enabled() {
         return (bool)$this->config->fileindexing;
     }
 
-    /**
-     * Defragments the index.
-     *
-     * @return void
-     */
-    public function optimize() {
-        $this->get_search_client()->optimize(1, true, false);
-    }
-
     /**
      * Deletes the specified document.
      *