Remove advice to use NCPUs jobs

This is generally too many, because cargo and tests can start so many
threads.

book/src/ci.md

@@ -29,37 +29,3 @@ jobs:
 
 The workflow used by cargo-mutants on itself can be seen at
 <https://github.com/sourcefrog/cargo-mutants/blob/main/.github/workflows/mutate-self.yaml>.
-
-## Parallelism
-
-If you'd like cargo-mutants to run in parallel (see [parallelism](parallelism.md) for more information) in CI, you can do the following:
-
-```yml
-name: cargo-mutants
-
-on: [pull_request, push]
-
-jobs:
-  cargo-mutants:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Get number of CPU cores
-        uses: SimenB/github-actions-cpu-cores@v1
-        id: cpu-cores
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: stable
-      - name: Install cargo-mutants
-        run: cargo install --locked cargo-mutants
-      - name: Run mutant tests
-        run: cargo mutants --jobs ${{ steps.cpu-cores.outputs.count }} -- --all-features
-      - name: Archive results
-        uses: actions/upload-artifact@v3
-        if: failure()
-        with:
-          name: mutation-report
-          path: mutants.out
-```
-
-This will automatically use the maximum number of CPU cores available. However, it's possible that this will result in flakiness due to OOM errors in the VM.