Remove libc++ hack

[alevenberg]

Remove hack added in https://github.com/googleapis/google-cloud-cpp/pull/14151.

During our Fedora 40 upgrades (https://github.com/googleapis/google-cloud-cpp/issues/14076), we found lib C++ broke the ABI to enforce C++ compliance (https://github.com/llvm/llvm-project/commit/d2cb198f25c82bf77bb113763771590cc79a21a4).

This means they removed a constructor for a std::future_error and the public constructor is only available in C++ 17 (https://en.cppreference.com/w/cpp/thread/future_error/future_error). When we upgrade to C++17, we can remove this hack.