nixos/atop: Remove upgraded logfiles if not replaced

Before this change, starting atop regularly caused a quadratic
increase in the number of log files over time, as each daily log file
was copied (multiple times) to a new file, and then left there because
the upgrade was a no-op.  This eventually led to atop being unable to
start because the log file name became too long!

nixos/modules/programs/atop.nix

@@ -150,6 +150,8 @@ in
                   if ! ${pkgs.diffutils}/bin/cmp -s "$logfile" "$logfile".new
                   then
                     ${pkgs.coreutils}/bin/mv -v -f "$logfile".new "$logfile"
+                  else
+                    ${pkgs.coreutils}/bin/rm -f "$logfile".new
                   fi
                 done
               '';