Now, edit your /var/lib/portage/world file. Look for catpkgs (ie. "category/packagename") that you no longer use or need on your system and remove them. Also consider packages you may have installed
with --oneshot that are not in the world file but should be, and add them. Portage will use /var/lib/portage/world as the master list of packages that should be on
your system. We will now look into cleaning up any unnecessary packages that are not in the world set. We want to remove these packages for a couple of reasons. First,
they will not get upgraded with a @world update. Second, because they are not included in @world, they could be outdated and have old and problematic dependencies that could hamper our upgrade, since portage will not want to "break" dependencies for these orphaned packages. Third, when we do an emerge @preserved-rebuild, we may end up rebuilding packages that we don't need. So removing unnecessary packages is a good idea for quite a few reasons.