[libc++] Stop copying headers to the build directory (#115380)

This was needed before https://github.com/llvm/llvm-project/pull/115077
since the compiler-rt test build made assumptions about the build
layout of libc++ and libc++abi, but now they link against a local
installation of these libraries so we no longer need this workaround.

GitOrigin-RevId: 428c8767ae997b0f726c0b40160ea8172551babf