archwiki: remove $wgFragmentMode whose default value changed in MW 1.37

$wgFragmentMode = [ 'html5', 'legacy' ]; is the default value since
MediaWiki 1.37:
https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:$wgFragmentMode

roles/archwiki/templates/LocalSettings.php.j2

@@ -81,9 +81,6 @@ $wgRunJobsAsync = 1;
 $wgAllowUserJs = true;
 $wgAllowUserCss = true;
 
-# Section anchors: use HTML5-style anchors as primary, legacy as fallbback
-$wgFragmentMode = [ 'html5', 'legacy' ];
-
 
 ##
 ## Database settings