Replace hard-coded 'Solr' with method call.

- Makes module more flexible/reusable.

module/VuFind/src/VuFind/Related/Similar.php

@@ -72,8 +72,9 @@ class Similar implements RelatedInterface
      */
     public function init($settings, $driver)
     {
-        $this->results
-            = $this->searchService->similar('Solr', $driver->getUniqueId());
+        $this->results = $this->searchService->similar(
+            $driver->getSourceIdentifier(), $driver->getUniqueId()
+        );
     }
 
     /**