Remove `-n 1` argument from the xargs invocation in blackbox_shred_all_files.
Remove `-n 1` argument from the xargs invocation in blackbox_shred_all_files.

The -I and -n options are mutually-exclusive, don't work as
expected with xargs from SunOS, and appear to be unecessary anyway.

unixorn/blackbox,StackExchange/blackbox,StackExchange/blackbox,unixorn/blackbox