[c++] Remove outdated TODO
[c++] Remove outdated TODO

We no longer need GMS Core hack

PiperOrigin-RevId: 291482238

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