texlive/combine.nix: clarify implementation of mkUniqueOutPaths

mkUniqueOutPaths used to produce empty paths for dummy packages, this
version strips those out. This does not affect `pkgList.bin` at all, but
`pkgList.nonbin` is affected, so this is not exactly a refactoring. It
should not harm to have a cleaner `paths`.

Also, original comment said "here we deal with those dummy packages
needed for hyphenation filtering". This doesn't seem to be true, the
packages that were really filtered are actually metapackages that
represent collections. I also could not find any dummy packages even in
the originally committed version.