Most of the materials we use are specially designed for going into space, so none of it is “unwanted”. In fact a lot of it is quite expensive to manufacture so none of it goes to waste! Occasionally we order extras of things, like electronic components or shielding foil, when we’re building an instrument. We save the extras and then use them in later instrument builds.
When we design things we often:
* use the best,
* or the cheapest,
* or the lightest,
* or the strongest.
There is always a reason for picking the materials we do. So in that sense nothing is unwanted.
There are some cases I think. Like when we design electronics, if we have a large stock of a certain component and we want to use them instead of throwing them away, we will make a compromise and design it in to save cost. In away they are unwanted materials.