[server] Don't set a parent on BufferInterface

BufferInterface used to have a SurfaceInterface as parent. This could
result in easily hitting the abort condition that the BufferInterface
was still referenced when it gets deleted by just having one other
user referencing the BufferInterface.

There is no need to have the BufferInterface deleted when the
SurfaceInterface to which it belongs gets deleted. The BufferInterface
will get deleted once it's completely unreferenced and also has a
destroy listener.