Exceptions in Joomla\CMS\Table\Usergroup refer to categories (#21098)

* Exceptions in Joomla\CMS\Table\Usergroup refer to categories

Pull Request for Issue #21092 .

### Summary of Changes
remove todo that is clearly a copy paste error from somewhere else
change exception message to usergroups from catergory

NOTE the first exception` if ($this->id == 0)`
I am not sure if that should even be present - doesn't look like it to me - please advise

* Update Usergroup.php

libraries/src/Table/Usergroup.php

@@ -160,12 +160,12 @@ public function delete($oid = null)
 
 		if ($this->id == 0)
 		{
-			throw new \UnexpectedValueException('Global Category not found');
+			throw new \UnexpectedValueException('Usergroup not found');
 		}
 
 		if ($this->parent_id == 0)
 		{
-			throw new \UnexpectedValueException('Root categories cannot be deleted.');
+			throw new \UnexpectedValueException('Root usergroup cannot be deleted.');
 		}
 
 		if ($this->lft == 0 || $this->rgt == 0)
@@ -189,9 +189,6 @@ public function delete($oid = null)
 			throw new \UnexpectedValueException('Left-Right data inconsistency. Cannot delete usergroup.');
 		}
 
-		// Delete the category dependencies
-		// @todo Remove all related threads, posts and subscriptions
-
 		// Delete the usergroup and its children
 		$query->clear()
 			->delete($db->quoteName($this->_tbl))