[libc++abi] Stop copying headers to the build directory

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.

Reviewed By: ldionne

Pull Request: https://github.com/llvm/llvm-project/pull/115086

GitOrigin-RevId: 5be02d7a03c6d40d4d71264936d4aab98e4186aa