MDL-39449 course view.php: don't require forum lib.php

As far as I can see, it is not used, except in social course format, so
I moved the include there.

course/format/social/format.php

@@ -2,6 +2,8 @@
       // format.php - course format featuring social forum
       //              included from view.php
 
+    require_once($CFG->dirroot.'/mod/forum/lib.php');
+
     $strgroups  = get_string('groups');
     $strgroupmy = get_string('groupmy');
     $editing    = $PAGE->user_is_editing();

course/view.php

@@ -4,7 +4,6 @@
 
     require_once('../config.php');
     require_once('lib.php');
-    require_once($CFG->dirroot.'/mod/forum/lib.php');
     require_once($CFG->libdir.'/conditionlib.php');
     require_once($CFG->libdir.'/completionlib.php');