[libc] [libcxx] [llvm] Remove last few bits for Native Client support (PR #148983)

[Brad Smith via llvm-commits]

brad0 wrote:

@ldionne I didn't remove the bits from the __cxx03 headers. I saw elsewhere that they're frozen and should not be touched. How will they eventually be cleaned up?

https://github.com/llvm/llvm-project/pull/148983