ci: drop third build

Author: henryiii

.github/workflows/ci.yml

@@ -202,24 +202,6 @@ jobs:
     - name: C++ tests
       run: cmake --build build2 --target cpptest
 
-    # Third build - C++17 mode with unstable ABI
-    - name: Configure (unstable ABI)
-      run: >
-        cmake -S . -B build3
-        -DPYBIND11_WERROR=ON
-        -DPYBIND11_PYTEST_ARGS=-v
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=17
-        -DPYBIND11_INTERNALS_VERSION=10000000
-        ${{ matrix.args }}
-
-    - name: Build (unstable ABI)
-      run: cmake --build build3 -j 2
-
-    - name: Python tests (unstable ABI)
-      run: cmake --build build3 --target pytest
-
     - name: Interface test
       run: cmake --build build2 --target test_cmake_build