`AbstractCommentManager` no longer inherits from `SingletonFactory`

`AbstractCommentManager` no longer inherits from `SingletonFactory`

The dependency on `SingletonFactory` offers no added value, as the comment manager is not accessed directly and it is only instantiated once per object type in `ObjectType::getProcessor()`. At the same time, however, the dependency on `SingleFactory` prevents a `CommentManager` implementation from being used in different/multiple object types.

ref https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/data/object/type/ObjectType.class.php#L109