Delete Firestore/cpp and public C++ artifacts

It's not yet feasible to build these components externally so there's no use in keeping them here.
firebase · wilhuff · Nov 7, 2018 · Nov 5, 2018 · Nov 5, 2018 · Nov 5, 2018
commit 5968113003197f10c315cd859ef2ee9a61d72ecb
diff --git a/Firestore/cpp/.clang-format b/Firestore/cpp/.clang-format
diff --git a/Firestore/cpp/.clang-tidy b/Firestore/cpp/.clang-tidy
diff --git a/Firestore/cpp/CMakeLists.txt b/Firestore/cpp/CMakeLists.txt
diff --git a/Firestore/cpp/include/firebase/firestore/CMakeLists.txt b/Firestore/cpp/include/firebase/firestore/CMakeLists.txt
diff --git a/Firestore/cpp/include/firebase/firestore/collection_reference.h b/Firestore/cpp/include/firebase/firestore/collection_reference.h
diff --git a/Firestore/cpp/include/firebase/firestore/document_change.h b/Firestore/cpp/include/firebase/firestore/document_change.h