for a registered 4 seater you can strip out the seats, and either take the belts out or leave them in
as Ian stated if there is seats there they need belts, but not vice versa
however if you need to register a car ie : an import, if it has no seats then it will get registered as a 2 seater, the belts will need to be removed and the holes for belts and seats must be welded up
I own a TRD msb vitz which is registered as a 2 seater as it never came with rear seats :rockon:
insurance wise i would think it may be best to inform them, although who is to say that you removed them for cleaning !!