removes check for obsolete feature from "supports" callback.

[stopfstedt]

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

=== 2.8 ===

• Constant FEATURE_GROUPMEMBERSONLY is deprecated. Modules should remove this
  constant from their module_supports() API function.
• $CFG->enablegroupmembersonly no longer exists.

this fixes #127

[davosmith]

Thanks - nice simple bit of clean-up (doesn't really affect anything much, but always good to clear out redundant old code).