btrfs-progs: tests: Remove misleading BCP 78 boilerplate from SHA implementation

btrfs-progs: tests: Remove misleading BCP 78 boilerplate from SHA implementation

BCP 78 applies to RFC 6234, but sha224-256.c is Simplified BSD.

This causes the following lintian error when building on Debian and
Debian derivatives:

    E: btrfs-progs source: license-problem-non-free-RFC-BCP78
       tests/sha224-256.c

Please consult the following email from debian-legal@lists.debian.org
for more information:

https://lists.debian.org/debian-legal/2017/08/msg00004.html
Signed-off-by: Nicholas D Steeves <nsteeves@gmail.com>
[ I've copied too much from the RFC that's not related to the code,
  covered by the explicit copyright notice in the file ]
Signed-off-by: David Sterba <dsterba@suse.com>