[libcxx] [ci] Stop manually installing ninja in the Windows build jobs

Ninja is officially included among the preinstalled tools on
the Windows runners now.

This should reduce the risk for stray failures here; sometimes,
attempting to install Ninja through Chocolatey have caused
spurious failures.

Author: mstorsjo

.github/workflows/libcxx-build-and-test.yaml

@@ -255,7 +255,6 @@ jobs:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
       - name: Install dependencies
         run: |
-          choco install -y ninja
           pip install psutil
       - name: Install a current LLVM
         if: ${{ matrix.mingw != true }}