hashsum: remove b3sum (or determine upstream) to avoid confusion

[oech3]

uutils's b3sum is clearly not a drop-in replacement for https://github.com/BLAKE3-team/BLAKE3 according to --help.
Please remove it before some confusion happen.

[oech3]

For the case someone decided to make a drop-in at here: https://manpages.debian.org/testing/b3sum/b3sum.1.en.html