Remove unused language files and corresponding functions

There are two functions in "\phpbb\search\base.php":
get_synonyms() and get_ignore_words()

They are not used since phpBB 3.0 (including 3.1 and 3.2). They were last used in phpBB 2.0 and transfered to phpBB 3.0, but never used again.

To this functions belong to language files:

• language\en\search_ignore_words.php
• \language\en\search_synonyms.php

So if nothing uses them, i think we should delete the functions and the two language files.