[llvm] [Support/BLAKE3] Remove some dead stores (PR #95120)

[Argyrios Kyrtzidis via llvm-commits]

akyrtzi wrote:

I'd like that we keep the changes with upstream BLAKE3 to a minimum, could you open a PR against https://github.com/BLAKE3-team/BLAKE3? Don't merge this until the BLAKE3 upstream PR is accepted and merged.

https://github.com/llvm/llvm-project/pull/95120