Revert "Add BLAKE3 support in Digest"

[byroot]

Reverts #133

This causes two issues in Ruby.

One with clang and -std=c99:

    /github/workspace/src/ext/digest/blake3/blake3_dispatch.c:108:5: error: '_Atomic' is a C11 extension [-Werror,-Wc11-extensions]
      108 |     ATOMIC_INT g_cpu_features = UNDEFINED;
          |     ^
    /github/workspace/src/ext/digest/blake3/blake3_dispatch.c:34:20: note: expanded from macro 'ATOMIC_INT'
       34 | #define ATOMIC_INT _Atomic int
          |                    ^
    1 error generated.

One on i686:

[42](https://github.com/ruby/ruby/actions/runs/31799715641/job/94764571088?pr=18331#step:15:448)
  /home/runner/work/ruby/ruby/src/tool/lib/test/unit/parallel.rb: TestDigest::TestBLAKE3#test_alignment: symbol lookup error: /home/runner/work/ruby/ruby/build/.ext/i686-linux-gnu/digest/blake3.so: undefined symbol: blake3_compress_in_place_sse41
  running file: /home/runner/work/ruby/ruby/src/test/digest/test_digest.rb

cc @knu @tenderlove