[tooling] leave kialo target url setting empty by default so that the…

… ENV var is the default but can still be overridden.

settings.php

@@ -64,7 +64,7 @@
             'mod_kialo/kialourl',
             new lang_string('kialourl', 'mod_kialo'),
             new lang_string('kialourl_desc', 'mod_kialo'),
-            'https://www.kialo-edu.com',
+            '',  // If left blank, this defaults to the TARGET_KIALO_URL env var or 'https://www.kialo-edu.com'.
         );
 
         $settings->add($kialourl);