Fix incorrect destruction of Outliner tree-elements

`TreeElement` isn't a trivial type anymore, so `MEM_delete()` should be
called, which calls the destructor.
AFAICS this would cause a memory leak, since the contained `unique_ptr`
is allocated but not destructed correctly - but it's not using the
guarded allocator so woudn't be reported.

source/blender/editors/space_outliner/outliner_tree.cc

@@ -218,7 +218,7 @@ void outliner_free_tree_element(TreeElement *element, ListBase *parent_subtree)
     MEM_freeN((void *)element->name);
   }
   element->type = nullptr;
-  MEM_freeN(element);
+  MEM_delete(element);
 }
 
 /* ********************************************************* */