Drop incremental-mutants CI job

Previously, the `incremental-mutants` CI job was failing on ~every PR that made actual logic changes, and nobody seemed to really make any effort to address the failures. The failing CI jobs therefore just resulted in additional which in turn could have us getting used to failing CI, introducing some risk of acutal failures slipping through. Of course, it also took up some (considerable?) time in the CI queue that might be better spent on other jobs if no contributors are actually benefitting from the CI job.

Here we therefore drop `incremental-mutants` from our CI for the time
being.

Author: tnull

.github/workflows/build.yml

@@ -301,20 +301,3 @@ jobs:
           rustup component add rustfmt
       - name: Run rustfmt checks
         run: cargo fmt --check
-
-  incremental-mutants:
-    runs-on: ubuntu-latest
-    if: github.ref_name != 'main' # `main` has no diff with itself
-    steps:
-      - uses: actions/checkout@v4
-        with:
-          fetch-depth: 0
-      - name: Relative diff
-        run: |
-          git branch -av
-          git diff origin/main.. | tee git.diff
-      - uses: Swatinem/rust-cache@v2
-      - name: Mutants
-        run: |
-          cargo install cargo-mutants
-          cargo mutants --no-shuffle -j 2 -vV --in-diff git.diff