[llvm] [libcxx] [ci] Stop manually installing ninja in the Windows build jobs (PR #161907)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

This might be left over from before the libc++ CI was running inside Github hosted runners?

I guess this makes the workflow less portable if someone wanted to try getting it running again on more powerful windows machines, but given the instability, this seems like a decent enough option for now.

https://github.com/llvm/llvm-project/pull/161907