github: Stop manually installing ninja on Windows

Since https://github.com/actions/runner-images/pull/11706,
https://github.com/actions/runner-images/pull/11707 and
https://github.com/actions/runner-images/pull/11737, ninja is
preinstalled in the Ubuntu, macOS and x86 Windows runner images.
Thanks to that, redundant installs of ninja on Unix were removed
in e20f0e3acd.

As noted in https://github.com/actions/partner-runner-images/issues/96,
ninja now also seems to be available on the Windows on ARM
images - thus remove redundant installs of ninja on Windows.

.github/workflows/test-llvm.yml

@@ -48,9 +48,6 @@ jobs:
           - aarch64
     runs-on: ${{startsWith(matrix.arch, 'a') && 'windows-11-arm' || 'windows-latest'}}
     steps:
-      - name: Install dependencies
-        run: |
-          choco install ninja
       - name: Download toolchain
         uses: dawidd6/action-download-artifact@v10
         with: