MDL-66118 hub: Drop support for the hub's display_homepage callback

There should never be any exceptional hacks like this for any particular
plugin. The hub sites can make use of a wide range of other and cleaner
mechanisms - such as a custom theme or the customfrontpageinclude
directive - to implement a customized front page.

Plus, we've dropped support for alternative hubs anyway.

index.php

@@ -91,18 +91,6 @@
 // Trigger event.
 course_view(context_course::instance(SITEID));
 
-// If the hub plugin is installed then we let it take over the homepage here.
-if (file_exists($CFG->dirroot.'/local/hub/lib.php') and get_config('local_hub', 'hubenabled')) {
-    require_once($CFG->dirroot.'/local/hub/lib.php');
-    $hub = new local_hub();
-    $continue = $hub->display_homepage();
-    // Function display_homepage() returns true if the hub home page is not displayed
-    // ...mostly when search form is not displayed for not logged users.
-    if (empty($continue)) {
-        exit;
-    }
-}
-
 $PAGE->set_pagetype('site-index');
 $PAGE->set_docs_path('');
 $editing = $PAGE->user_is_editing();