ci: remove x86-64 test for non-SSE environment

This was added in #77 to catch regressions, but it has starting
[failing on CI] in a way that I don't understand. Since this is a niche
use case, I don't feel compelled to spend the time investigating. On #77,
I suggested that we have one of three directions forward:

1. Support for testing this in CI needs to become more robust.
2. Its failure modes need to be better understandable by me.
3. We don't test it at all and instead rely on patches from
stakeholders to fix things if they break. (This is the new status quo.)

[failing on CI]: https://github.com/BurntSushi/memchr/actions/runs/8030404672/job/21937751272

.github/workflows/ci.yml

@@ -162,22 +162,6 @@ jobs:
     - name: Run tests without any features enabled (core-only)
       run: cargo test --verbose --no-default-features
 
-  # This job uses a custom target file to build the memchr crate on x86-64
-  # but *without* SSE/AVX target features. This is a somewhat strange
-  # configuration, but it pops up now and then. Particularly in kernels that
-  # don't support SSE/AVX registers.
-  build-for-x86-64-but-non-sse-target:
-    runs-on: ubuntu-latest
-    steps:
-    - name: Checkout repository
-      uses: actions/checkout@v3
-    - name: Install Rust
-      uses: dtolnay/rust-toolchain@master
-      with:
-        toolchain: nightly
-        components: rust-src
-    - run: cargo build -Z build-std=core --target=src/tests/x86_64-soft_float.json --verbose --no-default-features
-
   # This job runs a stripped down version of CI to test the MSRV. The specific
   # reason for doing this is that dev-dependencies tend to evolve more quickly.
   # There isn't as tight of a control on them because, well, they're only used