Remove unnecessary cplusplus flag from being defined for Windows builds
Remove unnecessary cplusplus flag from being defined for Windows builds

The flag was added because move operators were not enabled on some versions of visual studio without it, however cl/274675656 fixed the issue so that it will work without the flag.

PiperOrigin-RevId: 288730959

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk