removes check for obsolete feature from "supports" callback.

this feature check can be safely removed without replacement at this
point.
Moodle 2.8 was a long time ago, this plugin supports Moodle 4.1 and up.
For reference, check the "2.8" section in the upgrade note for activity
plugins, here: https://github.com/moodle/moodle/blob/main/mod/upgrade.txt

lib.php

@@ -884,12 +884,6 @@ function checklist_supports($feature) {
         }
     }
 
-    if ((int)$CFG->branch < 28) {
-        if ($feature === FEATURE_GROUPMEMBERSONLY) {
-            return true;
-        }
-    }
-
     switch ($feature) {
         case FEATURE_GROUPS:
             return true;