Remove all traces of AudioBook, clean up some warnings

Lib/db/DbMapping.php

@@ -101,15 +101,6 @@ abstract class DbMapping
         $this->attributes[$name] = $value;
     }
 
-    /**
-     * Function to retrieve data from an id.
-     * @param int $id
-     * @return DbMapping
-     */
-    public static function find($id) {
-        throw new \RuntimeException("This method must be implemented in child classes.");
-    }
-
     /**
      * Return all the public attributes in an array;
      */