docs: remove incorrect claim re gc --print-dead

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 7ab6896 (* 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.

Author: me-and

doc/manual/source/command-ref/nix-store/gc.md

@@ -48,8 +48,7 @@ The behaviour of the collector is also influenced by the
 configuration file.
 
 By default, the collector prints the total number of freed bytes when it
-finishes (or when it is interrupted). With `--print-dead`, it prints the
-number of bytes that would be freed.
+finishes (or when it is interrupted).
 
 {{#include ./opt-common.md}}