Remove removeBookById from the Manager.

Use the same method of the `Library`.

include/manager.h

@@ -99,14 +99,6 @@ class Manager
    */
   bool readOpds(const string& content, const std::string& urlHost);
 
-  /**
-   * Remove a book from the library.
-   *
-   * @param id the id of the book to remove.
-   * @return True if the book were in the library.
-   */
-  bool removeBookById(const string id);
-
   /**
    * Set the path of the external fulltext index associated to a book.
    *