Remove workarounds for building upb on Windows

Other issues:
https://github.com/protocolbuffers/protobuf/issues/17032
https://github.com/grpc/grpc/issues/36815
https://github.com/microsoft/vcpkg/issues/39177

We have to work around the 14.40.* VC toolsets in order to run our Windows CI builds. This issue tracks removing the workaround.