remove redundant parentGroup

there was already a parent BelongsTo...

Author: bpow

app/Modules/Group/Models/Group.php

@@ -280,11 +280,6 @@ protected static function newFactory()
         return new GroupFactory();
     }
 
-    public function parentGroup(): BelongsTo
-    {
-        return $this->belongsTo(Group::class, 'parent_id', 'id');
-    }
-
     public function representationForDataExchange(): array {
         $item = [
             'id' => $this->uuid,