Per #7591, the `nix-store --gc --print-dead` command does not provide
any feedback about the amount of disk space that is used by dead store
paths. It looks like this has been the case since 7ab68961e (* Garbage
collector: added an option `--use-atime' to delete paths in...,
2008-09-17).
Update the nix-store documentation to remove the claim that this is
function that `nix-store --gc --print-dead` performs.
972915cabd