remove clang 13 CI since the install method isn't working anymore and…

… we have another test for clang

.github/workflows/build_cpp.yml

@@ -195,31 +195,6 @@ jobs:
     - name: Build examples, etc
       run: cmake --build build --config Release --parallel 2
 
-  ubuntu-latest-clang-13:
-    runs-on: 'ubuntu-latest'
-    steps:
-    - uses: actions/checkout@v2
-    - name: Install dependencies
-      run: |
-        sudo apt update
-        sudo apt install libwebp-dev libavformat-dev libavcodec-dev libavdevice-dev libavfilter-dev libswresample-dev libswscale-dev libavutil-dev 
-    - name: Install clang 13
-      run: |
-        wget https://apt.llvm.org/llvm.sh
-        chmod +x llvm.sh
-        sudo ./llvm.sh 13
-    - name: Configure
-      run: |
-        export CC=/usr/bin/clang-13
-        export CXX=/usr/bin/clang++-13
-        cmake ${{ github.workspace }}/dlib/test -B build
-    - name: Build just tests
-      run: cmake --build build --config Release --target dtest --parallel 4
-    - name: Test
-      run: build/dtest --runall -q
-    - name: Build examples, etc
-      run: cmake --build build --config Release --parallel 2
-
   windows-latest:
     runs-on: 'windows-latest'
     steps: