MDL-73526 mod_wiki: Remove unused nwiki diff processor

It never has been used in core since it was introduced in 2011
by MDL-22112, because the ouwiki_diff_html() is used unconditionally
for all wiki formats (html, creole or nwiki).

So, removing it to avoid any current (php80) or future incidence
with it.

mod/wiki/diff.php

@@ -38,7 +38,6 @@ require_once($CFG->dirroot . '/mod/wiki/locallib.php');
 require_once($CFG->dirroot . '/mod/wiki/pagelib.php');
 
 require_once($CFG->dirroot . '/mod/wiki/diff/difflib.php');
-require_once($CFG->dirroot . '/mod/wiki/diff/diff_nwiki.php');
 
 $pageid = required_param('pageid', PARAM_TEXT);
 $compare = required_param('compare', PARAM_INT);