github/actions: Remove clang9 on Windows

This currently produces

 In file included from C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.27.29110\include\ciso646:9:
 In file included from C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.27.29110\include\yvals.h:9:
 C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.27.29110\include\yvals_core.h:494:2: error: STL1000: Unexpected compiler version, expected Clang 10.0.0 or newer.
  #error STL1000: Unexpected compiler version, expected Clang 10.0.0 or newer.

and that is very likely due to the new -pedantic flag when compiling.

Let's remove this compiler as it clearly is not meant to be used with
these MSVC headers.

.github/workflows/windows.yml

@@ -15,20 +15,6 @@ jobs:
     - name: test
       run: cd build ; ctest -j 10 -C Debug --exclude-regex "test-unicode" --output-on-failure
 
-  clang9:
-    runs-on: windows-latest
-
-    steps:
-      - uses: actions/checkout@v1
-      - name: install Clang
-        run: curl -fsSL -o LLVM9.exe https://releases.llvm.org/9.0.0/LLVM-9.0.0-win64.exe ; 7z x LLVM9.exe -y -o"C:/Program Files/LLVM"
-      - name: cmake
-        run: cmake -S . -B build -DCMAKE_CXX_COMPILER="C:/Program Files/LLVM/bin/clang++.exe" -G"MinGW Makefiles" -DCMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On
-      - name: build
-        run: cmake --build build --parallel 10
-      - name: test
-        run: cd build ; ctest -j 10 -C Debug --exclude-regex "test-unicode" --output-on-failure
-
   clang10:
     runs-on: windows-latest