Deleting unused or unwanted parts from a score

https://musescore.org/en/handbook/measure-operations#delete
or
In the Parts window click Delete to delete a part
Press 'I' and remove the Instrument?

To condense...
For instruments not currently playing (the staff has no music), you can hide empty staves.
See:
https://musescore.org/en/handbook/layout-and-formatting#style-edit-gene…

You can also create a part for a single instrument, or a combination of instruments (e.g. all the woodwinds).
See:
https://musescore.org/en/handbook/parts
and:
https://musescore.org/en/handbook/parts#setup-parts

Regards.