ci: remove the deadsnakes job (#6052)

Drop the last remaining deadsnakes-based Linux job because it mostly duplicates the main Ubuntu coverage while failing in external Launchpad/PPA setup, and the old Valgrind/debug path it used to complement has already been retired.

Co-authored-by: Cursor <cursoragent@cursor.com>

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c649a3f..6556ebd 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -279,87 +279,6 @@
       - name: Python tests C++11
         run: cmake --build --preset testsvenv -t pytest
 
-  deadsnakes:
-    if: github.event.pull_request.draft == false
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-        # TODO: Fails on 3.10, investigate
-        # JOB DISABLED (NEEDS WORK): https://github.com/pybind/pybind11/issues/4889
-        # - python-version: "3.9"
-        #   python-debug: true
-        #   valgrind: true
-        - python-version: "3.11"
-          python-debug: false
-
-    name: "🐍 ${{ matrix.python-version }}${{ matrix.python-debug && '-dbg' || '' }} (deadsnakes)${{ matrix.valgrind && ' • Valgrind' || '' }} • x64"
-    runs-on: ubuntu-latest
-    timeout-minutes: 90
-
-    steps:
-    - uses: actions/checkout@v6
-
-    - name: Setup Python ${{ matrix.python-version }} (deadsnakes)
-      uses: deadsnakes/action@v3.2.0
-      with:
-        python-version: ${{ matrix.python-version }}
-        debug: ${{ matrix.python-debug }}
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v2.2
-
-    - name: Valgrind cache
-      if: matrix.valgrind
-      uses: actions/cache@v5
-      id: cache-valgrind
-      with:
-        path: valgrind
-        key: 3.16.1 # Valgrind version
-
-    - name: Compile Valgrind
-      if: matrix.valgrind && steps.cache-valgrind.outputs.cache-hit != 'true'
-      run: |
-        VALGRIND_VERSION=3.16.1
-        curl https://sourceware.org/pub/valgrind/valgrind-$VALGRIND_VERSION.tar.bz2 -o - | tar xj
-        mv valgrind-$VALGRIND_VERSION valgrind
-        cd valgrind
-        ./configure
-        make -j 2 > /dev/null
-
-    - name: Install Valgrind
-      if: matrix.valgrind
-      working-directory: valgrind
-      run: |
-        sudo make install
-        sudo apt-get update
-        sudo apt-get install ninja-build libc6-dbg
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt
-
-    - name: Configure
-      run: cmake --preset default -DCMAKE_CXX_STANDARD=17
-
-    - name: Build
-      run: cmake --build --preset default
-
-    - name: Python tests
-      run: cmake --build --preset default --target pytest
-
-    - name: C++ tests
-      timeout-minutes: 3
-      run: cmake --build --preset default --target cpptest
-
-    - name: Visibility test
-      run: cmake --build --preset default --target test_cross_module_rtti
-
-    - name: Run Valgrind on Python tests
-      if: matrix.valgrind
-      run: cmake --build --preset default --target memcheck
-
-
   # Testing on clang using the excellent silkeh clang docker images
   clang:
     if: github.event.pull_request.draft == false